You may want to hang on to the FX, since they aren't made anymore. They are very strong, reliable engines, and hard to beat. You most likely won't get much for it used, but I would keep it for later projects, or back up for the AX. I believe the tower muffler will fit the AX as well, but don't have any first hand experience. Maybe someone else can let you know. You definitely want the extra flow this muffler gives .46 to .52 engines that it is compatible with.

Make sure you beef up your fuselage on the UCD, if you fly off of grass. It is weak and will eventually break out, but is easily repaired and made stronger after it gives way.

Happy landings.