RE: Aircraft for an OS 91 4-stroke
Just about any 60 sized airplane that you like.
I used that very same engine on the following
Four Star 60 (great match)
Seagull Extra 300 (nice match)
Seagull Edge 540, (the previous version, decnt match)
Funtana90 (seriously lacking though in the power department but for general flying it was fine)
SIG Hog Bipe (kit though, perfect match)
Carl Goldberg Ultimate (also somewhat lacking, but still flew well)
I'm sure the Tiger 60 would be a good match
I've seen one in a Hangar9 60 Mustang and fly it well.