Outboards 20" or 25"

[tsubaki]

You'd burn it up the first outing.
Cavitation/ventilation.
You really need to either cut the transom or have a bracket.

I've not looked at the possibility of adding a fixed jack plate, mounted upside down (or backwards) in order to accommodate a short shaft on a 25" transom (as opposed to a long shaft adapted to a 20" transom). It's possible the 5 or so inches of setback might work. One definite problem would be the likely inability to tilt the motor up appreciably. The cost difference in a 20" motor plus the fixed jack plate would probably be the same as a 25" engine.