Aerobender,

When you say .91, I am hopefully assuming you mean a .91 2 stroke. There are no vintage designs (pre 1980), which happily take a .91 4 stroke. Sure, they will fit,(with a shoehorn and removing most of the nose), but you are opening a can of worms in weight/balance. Any of the "vintage" birds fly great on a simple OS or other good, present day .61... even without a pipe. Main reason is that the vintage ships are smaller, and designed to do big manuvers without the unlimited horsepower we have today. My choice would be to use an OS .91 fx, which is the same size and lighter than their .61. I'm getting ready to do one of those soon.. so will let you know how it goes.

Deadstik