The little Canonet is a fun camera. I briefly had the f/1.9 version, but I sold it off as I prefer aperture priority to shutter priority. Replaced it with an Olympus 35SP, that I then sold off when I realized I didn't really care about having a meter or an automated exposure mode. That said, the whole class of reasonably affordable small semi-automated cameras with great, fast prime lenses is something that really doesn't exist anymore, which is a shame.
Modern hearing aid batteries work fine. They don't last, but they are cheap and easily available. At least the battery compartment is accessible with film loaded on the Canonet (unlike my Rollei 35, but then I really don't use the meter anyway...still a fairly stupid design, though).