RE: 2 to 4 stroke?
I have a OS52fs on my LT40 trainer and it's a great combo, haven't flown off water with it, but I'm sure it'll work OK.
For a 50 size fourstroke for your trainer you can for example have a look at: OS52, Saito56 or Magnum52.
Fourstrokes have great idle, nice trottling response and that puuuring sound, go for it.