Johnson VS Evinrude - Compatibility???

I have one Johnson 65 hp and two Evinrude 65 hp outboards, all are from '72 to '73 and all look identical, are they??? Are the parts all interchangeable and compatible with one another? Need to know because I would like to make one great running beast between the three and if so, which repair/service manual should I purchase? One more probably stupid question, if they are all two-stroke then why are there three pistons and three coils on each?

Did my time in the Navy but have never owned my own boat, I am 50 yrs old and now learning a new hobby. Thanks for any ones help!!!

First, they are identical. The . My manual (Clymer)covers 1973 to 1990, so there may be some differences from 72-73. There were hydro-mechanical and electric shift models. Next, two stroke and 4 stroke has nothing to do with how many cylinders an engine has, it refers to compression and exhaust. A 2 stroke means compression and exhaust in each direction.A 4stroke compesses on one stroke and exhausts on the other.