Heres a perspective having built 2 TFP-51's with a third under way, Also the owner of 2 OS FX-91's. Read the thread under the engine section on all the troubles with the FX overheating. I tried to use the TF incowl muffler with a ST-90 . I drilled it out etc and it just couldnt work with a 90 size that runs cool, the FX runs hot, ( if you can get it to run at all). IMO your going to spend alot of time and $ building a great kit and than make a BIG mistake on the most important component. The best set up is a 120 Four stroke, I have both the OS pumped and nonpumped versions in my 51's and they work perfect! The head purtrudes alittle out the bottom, maybe half inch, and the muffler out the side of the lower starboard cowl but at 5 feet away you cant tell. Go to the pic. gallery and look at my 51 under Kirk RC. I wouldnt put the FX91 and TF incowl muffler in my 51's if you gave it to me for free, on the plus side it will make for a good pic in the crash section. Just a few thoughts, good luck, its a super warbird kit.
Kirk rc