1965 Johnson seahorse 40hp electric start is what I'm finding on the motor does that help?
First off,
Get rid of the engine and get a new one.
The engine is old and will need work done to it.
If you have to ask whether its a 2 or 4 stroke, then you have know idea what the difference is and you should not be allowed to pick up a wrench.
Just get something newer that doesn't need work and go enjoy the water.
Does anybody have a parts diagram for his engine? If it has all needle bearings then 50:1 should be fine, if there are any bushings, even in the wrist pins or just at the bottom of the crankshaft, you should run like 24:1 or even 20:1.
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Still if you have to ask that question, should you really be wrenching on an old motor?