I am pretty sure that the FS you see listed means Four Stroke. I don't have any experience using a four stroke on a stunt planes, but I seen it done. Mostly with the bigger engines. They are a lot heavier and not as powerful for their size.
Got to love those FP engines. I have 3 and they all run great. Too bad O.S. decided to make cheaper engines instead of keeping a better engine on the market.

Jay