RE: 4 stroke replacement
A OS 70 Surpass II would serve you well.
The Bridi trainer would be a little crowded up front for a 4 stroke? They were designed before the 4 stroke was. It could be set up a little different and made to work.
The engine would fit a 4 Star 40 fine but don't know about your plane?
The OS FL 70 and Ultimate 70 have not been as trouble free as the Surpass II version.
Enjoy,
Jim