The early 70's 20 hp could have either points type ignition ( and a short to spark points ignition at that), and as it went into the mid 70's a "Thunderbolt" Capacitive Discharge ignition.
That being said I would suppose if you found the exact model as you currently own, swapping it out should be a somewhat straight forward project.
Now envision that during the period from the sixties through the eighties they may have used basically the same block for a multitude of models and sizes with a few mounting tabs, bore size, or other incidentals changed around a bit. That might allow a person to mix and match parts pieces from who knows what to make a motor to use or sell. It's got to be close to 40 years now.
If you have a serial number, and it is applicable to the unit you have onhand, use it and buy what you need new. If you are just trying to piece/part it together from used parts "said to be from" this or that, good luck.
I don't think any of them came with a regulator. Many of theese smaller motors only had an ac output to run lights. To covert to DC a (less than) $10 rectifier from radio shack will work for that.
Don't expect more than a few amps at speed and nearly no output at idle, or trolling speeds.
Unregulated, as they are, or were, they can raise battery and system voltages into the 18+ volt range at cruise.
Now let me impart a bit of personal insight from a 1975-76 model I owned and used for a year or so.
If the CDI is weak, or eradic change it with new (about $300) If it's points type ignition and all burnt up looking it may be normal.
Mine took well over 7 "big man two hand" pulls to get it fired off cold, but would pop right off with half a pull when warmed. This is only with the motor attached to boat and operator in the motor starting position. If hung from a barrel or tried to start on a stand, the lack of support meant a brisk pull was not available and starting would be problematic.
Another issue with these motors is that as the lower unit is replaced, such as after an impeller change, and the nut tightened above the prop, it would pull through the casting leaving a large area for exhaust gasses to enter the prop wash and cavitate every time it come close to planing out.
I understand the actual starter motors for those are somewhat rare to find used.
So all in all I would say unless you find your exact motor with electric start to swap out parts from, forget it.
And if you expect it to run much more than a light bulb or two from the charging system, forget that one too.
I loved my 20hp Merc. I put on a 9" pitch prop, and spun it fast and easy. well over 20mph on a 14'er with my lardass aboard and a passenger too.
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Today's Featured Article - Third Brush Generators - by Chris Pratt. While I love straightening sheet metal, cleaning, and painting old tractors, I use every excuse to avoid working on the on the electrics. I find the whole process sheer mystery. I have picked up and attempted to read every auto and farm electrics book with no improvement in the situation. They all seem to start with a chapter entitled "Theory of Electricity". After a few paragraphs I usually close the book and go back to banging out dents. A good friend and I were recently discussing our tractor electrical systems when he stated "I figure it all comes back to applying Ohms Law". At this point
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