If it were me, (and I'm thankful that it hasn't been so far) I would go with twins. Since you're rebuilding the transom anyway you can widen it to accommodate twin engines and raise it up to use 25" instead of 20 shaft length units. I've often though that a pair of old 115 hp Merc Tower of Power inline 6 engines would go great on the stern of a V20. My thinking is that they are light compared to some engines, and just as importantly they are skinny compared to V4 and V6 Johnnyrudes of the same period. Oh, and they are substantially cheaper than a new engine. I had a 115 on one of my old boats a while back (18' Cruisers, Inc.) and I completely loved it. NOTHING else has the same growl when you're up on plane at WOT.
Understand this is purely dreaming, and I hope that I am never put into a position where it might become reality. Plus, finding a matched set of them in good condition might be a problem. But you asked for thoughts on the subject, and those are mine.
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1987 V20 w/1987 150HP Yamaha on a Shoreland'r Trailer
1978 16.5 Airslot w/1996 120HP Force on a Four Winns trailer
1996 V21 w/1993 200HP Mercury on a Shoreline Trailer
All towed by a 5.7L Hemi Durango.
If God didn't have a purpose for us we wouldn't be here, so
Live simply, Love generously, Care deeply, Speak kindly.
(Leave the rest to God) 
Silence, in the face of evil, is itself evil. Not to speak is to speak, not to act is to act. God will not hold us guiltless.
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