Hi all...2 strokes got phased out somewhat because they couldn't meet emissions.now they can get them as clean as 4 strokes.2 strokes are coming back.ktm has just released a new fuel injected motorcycle.2 strokes are more powerful and lighter with many less moving internal parts,so in theory 2 stroke should be more reliable.
if your going to repower with a smaller engine you should consider a modern 2 stroke,the bottom end torque will feel you've lost little to no power.
a mate of mine repowered his his trophy 21 a few years ago from a 200 2 stroke to a 140 or 150 4 stroke,the boat was a pig after the motor change so he got rid of it.he lost a lot of money on it,do your homework.