The recommended range is 91 to 1.25, 2 or 4 stroke. I would do a comparison between them and your 100 4 stroke. Just of hand I would think it would okay for sports flying. So I guess it depends on how you plan to fly it. Also consider the prop size, a lager prop with less pitch will allow you engine to run at higher speed. I had an Extra 300 the same size with an OS 91 4 stroke in it and that was more then adequate for me. As I stated before I’m going to put a SPE 26 in mine for the simple reason that I would like the power to do any type of flying. Plus gas is cheaper in the long run.
Other recommendation…you must be in a R/C club, ask one of the members that has the experience and you trust…look in the discussion board for threads that deals with engine sizes or start one in a different category other then this one.
The bottom line the decision is yours and that should be based on all the information you collect and the type of flying you want to do.
Good Luck and Happy Flying!
Aerosman