how much power do I needed?

[pogden]

My 1978 Master Angler originally had a very tired 1979 Evinrude 150, it ran about 35-38 mph. I am rebuilding the transom and have a 2001 Merc 200 EFI almost ready to go.

I think anything from a 135 to a 150 would be fine on a older 20" transom. If you want to rebuild the transom and raise it to 25" and maybe go with a bracket then you could probably go with the heavier(more HP and/or 4-stroke)motors.