I donlt know these machines, so this is a bit "high level". For what it;s worth the old (pre thru' prop exhaust) OMCs the 20 & 25 shared a powerhead, and I think when the thru' prop appeard may be when you are alking about.
Anyhow, the first thing to do is grab the briochuires for your chosen machines, and do a hioghlevel comparison of Bore, stroke, CC & weight. SOme brochires tell you the max RPM as well, depends o nthe vintage & OEM. The good thing is you are panning on using OMC, so most of what you'll need is on the old OMC site
So, if your two chosen engines are the same CC, weight, bore & stroke, then you are halfway there. If you are lucky & the brochure shows RPM as well, the chances are one wil lbe 1000 or so higher than the other.
So, having got your warm feeling that it might be feasible, the next stop is the microfiche for your two chosen engines. Go through it with a fine tooth comb. In order I'd check:
- Powerhead for any restrictor plates, differences in exhaust / inlet etc
- carb(s) At that vintage you may find it only has one, so itl;s either going to be a smaller bore or have a different wot stop
- Throttle linkage - this is another place you could restrict opening, and if it's on a stop there may be a different follower cam to give a more linear response.
It may of course just be down to timing, but pop back if you can find no differences in the parts lists. Also remember to compare like with like - the paint job is usually enough to change part numbers on outboards!