1965 Johnson 6 horse. Bullet proof motor (dang near all of the OMC twin cyl motors of that era and later were). Technically, you are supposed to run that motor at 50:1 (fuel
il), but seeing as those smaller motors still had bronze sleeve bearings in the upper end, most guys (myself included) would run that at 40:1.
Shouldn't take much at all to get that thing running again, due to the simplicity of the design. First, check for spark, and do a compression check. If your compression is bad, you may not want to put money into it.
If it doesn't have spark, take the flywheel off (use a flywheel puller, NOT tap it, like some manuals say. I have a stack of cracked flywheels in the outboard parts pile due to people not wanting to use a puller), and fold up a piece of 600 or higher grit sandpaper. Put those in between the points, and rub it back and forth a bit. Often, due to lack of use, the ends of the points will slightly corrode, and just need to be cleaned.
If you have spark then, put some fuel/oil in the tank, and try to fire it up in a bucket of water. Chances are, it will need a carb cleaning, but you never know. Do not use starting fluid in a 2 stroke. Instead, put some premix in a squirt bottle, and use that if you need to (do not spray while turning it over - just spray some in the intake before attempting to start - it could backfire into the bottle otherwise). If it doesn't start at all, I follow all my fuel lines, and clean the carb float bowl, and needle valves.
If you can get it to run, even for just a few seconds, you are good to go. I always recommend a carb kit, new water pump impeller and gear lube.
Why don't I just go ahead and put a new carb kit on before messing with it? I usually don't on these older cheaper ones, as it is cheaper to chunk it and buy another than to do major repairs on them. I collect older outboards, and love repairing them, but on most (certain rare ones I will put some $ into), I can find one without problems cheaper than I can fix that one. Sometimes, you don't find a major internal issue until you start trying to start it. Thus, I don't want to waste 30 bucks on a carb kit and impeller on a motor I will chunk. I always start it before putting new pieces. Usually, cleaning some gunk out of the float bowl and needle valve is all that it takes to get it running, even if it goes only a minute or so. That is all I need to hear.
That motor will likely outlast you if you care for it. They are very simple reliable designs that are proven to hold up extremely well. Not a bad price, even for a nonrunner, unless there is a major issue with it.