As regards the 24-25cc engines, the weak ones would work in the 91 size planes but a strong running engine would work in a 120 size plane.

There was a guy in Mansfield that put a Homelite 25 in a Venture 60. The plane flew and was not over powered but the plane was changed completely because of the extra weight of the engine. Sorta ruined a good Venture 60. There's a picture of the pink plane on the Venture site.

I have a Robinhood 80 that flies nicely with a OS 91 Surpass II but some people have both the old style and new style Zenoah G23's in them. They fly nice also.