4 strokes usually need to be bigger than 2 strokes to get equiv power but it's getting better.
Example: 2 stroke 60 = 4 stroke 90 approx
This also varies upon brand and model of both engines and is definitely a fixed ratio. Saito, Enya, Magnum, OS, YS all have different efficiency engines in different sizes of engines.

Mounting holes? Depends on the mounts you have. Important thing is to have the shaft end up in the same place when done. Not all mounts have a "square" mtg pattern. May be rectangular or even trapazoidal.