Just use exterior great plywood (not treated) and epoxy resin. The original lasted as long as it did with inferior polyester and no real encapsulation of the wood. Exterior grade with full epoxy encapsulation will outlast you. Anywhere you need to penetrate the deck, like seat mounting screws, drill out oversized, like an inch, and fill in with epoxy peanut butter, then drill for your mounting screws. This will prevent you from creating spots to wick moisture into the wood. It'll outlast you.
In reply to TommyWants :
Any specific wood types or grades to look for? I don't think my standard box stores have anything that matches your description and when I call the local lumber yards it would be nice to know what I'm asking for.
Edit: is BCX grade pine acceptable?
Marine grade ply unless you want to do it again and again.
Everywhere you drill a hold or drive a screw is a entry point for moisture. It takes no time before those screws pull out of the soggy wood and you are driving more screws and more moisture intrudes. You have to be committed to sealing the wood with epoxy or fiberglass and making sure every screw hole, drain hole is sealed as well. Must seal both sides of the marine ply. West Marine has some good stuff on their website about fiberglassing for boats. Read up first.
Yes, marine grade plywood. Make sure you strip back everything to where there's no rot - it's like fixing a rusty car, if you don't get it all it just comes back to haunt you later.
In reply to stuart in mn :
Don't waste money on Marine grade. Exterior grade uses the same water resistant glues and you don't need the void free structural benefits of Marine grade for this application. Just fully encapsulate it and seal or eliminate any points of penetration. Auraco ply is well liked but any decent exterior is fine.
And not treated. Avoid treated
MGP is typically made of hard woods while exterior is usually soft woods. I'd go with the MGP just based on that.
Join iboats forum and read through their rebuilds. You'll find good info from people who have tons of knowledge and experience.
And don't waste money on Marine grade. That's like using moly tubing as conduit in this case. Unless you're building an actual plywood boat. Glass boats strength is in the composite structure.
Old motor, to go with the old boat, time!
1961 Evinrude Lark III 40HP
A starter and a generator! So decadent.
Can't all be sunshine and roses, though. Need to find another head without the threads stripped. Alternatively find better thread inserts
In reply to Mr_Asa :
Might need a generator belt too. 
Cool old engine.
In reply to Toyman01 (Forum Supporter) :
And a carburetor knob, and a tune up, and an impeller, and a and a and a and a....
I like it. I wasn't 100% sure before I put it next to the boat, but the price was good so I grabbed it for now. Looking at them next to each other it is very 60s and I like it
Helicoil or timesert possible? Makes me jelly of a few things. You gots a 40hp, a starter, and a generator.
Also agree that it's a perfect fit for your boat style wise.
Been too long since I've done old boat stuff, but I remembered that Marine Engine has good part breakdowns of post-57 or so. I pulled the part number and was able to find a NOS head somewhere near Dayton, Oh for only $20.
Here's my engine. Find yours if you think you are missing anything or want to take anything apart. https://www.marineengine.com/parts/johnson-evinrude-parts.php?year=1961&hp=40&model=35525
Yours is a 25HP? If you're going near Austin anytime soon, I know a guy down there that has a late 50s Evinrude 35HP he saved from the dump. No clue what it needs, but he said he'd be happy to give it to someone who was interested in getting it on the water again rather than selling it for scrap
In reply to Mr_Asa :
Too far for a road trip I think. There's a few that are kinda iffy around me, so I'll keep my eyes peeled for one that fits the bill.
I figured it would be, but I thought I'd mention it in case you have friends or family in the area that could pick it up.
In reply to Mr_Asa :
I need more friends and family by the sound of it.
I know its not cheap, but this stuff is a 'do it once' solution.
In reply to CJ (It's Just a Flesh Wound) :
For the transom, this is potentially a good option. I was actually thinking about canvas based micarta board for the transom and just going with exterior or marine grade plywood for the flooring. I'll dig into it more, thanks
Okie doke. NOS head delivered, time to fix that stripped thread!
Wait...
E36 M3.
At least the two that broke flush allowed me to unscrew this one by spinning the head once I broke the seal loose.
Pepper helped and learned new words.
Didn't they make these near me in Grand Prairie Texas? I see a lot of them around here.
Can the dog work with plywood?
In reply to Snowdoggie :
Founded there, actually. I think they eventually moved locations though. http://www.fiberglassics.com/library/index.php?title=Lone_Star
If you see parts for this year and model, I may be interested. Shipping might be fun to figure out.
Unfortunately the only thing she actually works is the squirrels and lizards in the yard.
Mr_Asa said:In reply to Snowdoggie :
Founded there, actually. I think they eventually moved locations though. http://www.fiberglassics.com/library/index.php?title=Lone_Star
If you see parts for this year and model, I may be interested. Shipping might be fun to figure out.
Unfortunately the only thing she actually works is the squirrels and lizards in the yard.
I see them at charity auctions all the time. Sometimes entire boats that age will sell for less than $200. I will let you know if I find anything.
I have an old Bayliner that will need a new floor soon. My Border Collie already figured out that putting that much in a Bayliner is a waste of money. I would really like to find an old Glasstron GT to restore. Just like the ones in the Bond movie.
In reply to Snowdoggie :
Fiberglassics might be a good place to look, if you haven't already?
In reply to Mr_Asa :
Thanks. I haven't seen that one yet. I will spend a few hours there tonight.
Not that I need another boat project. I am up to my neck in projects right now. Did I tell you about my old Dodge Motorhome? 
Believe me, I understand. Mustang, Truck, three boats (one doesn't need anything,) a dozen antique outboards, an antique lathe, a 3D printer, and god knows what else.
Also, I painted and plasti-dipped my PVC trailer guides. Painted cause the sun will damage PVC eventually, and why not.
Yes, the plasti-dip line is even with the red on the boat on both sides, its the little things.
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