The lower unit on my 1969 Mercury 650 Red Band has a small hole from an impact from previous owner. Ideally I'd like to replace the lower unit: What models interchange with mine? There are plenty up here around Toronto Ontario, I just need to know which one to pick up.
Both 800-4 and 850-4 (along with the later 80hp and 75hp variants) will bolt right up and have the same 2.3:1 gear ratio. I can't recall for sure if yours has the 2.3:1 but I did have a '71 with that ratio and they run very well with those gears.
Craigslist is your friend for spare parts!
Last thought, if the hole isn't in the oil-containing parts of the gearcase, you could have it weld-repaired then repaint.