Do yourself a big favor and get a Tower muffler: (Tower # LXJB16). Another alternative is the Hobbico muffler fitting the same dimensions, stock # LXCLH6.

It sells for $15 and fits your OS FX (or the new AX) and you'll get 700 more rpm with virtually no increase in weight over a stock muffler. The only bad aspect I've found with that combo is you'll probably need to loc tite your carb screws and possibly the muffler ones, as they tend to vibrate out with the increased power.

Whatever plane you do get, I suggest keeping the total weight at or under 4.5 lbs. With your engine and this muffler you'll be able to hover at half stick with any 4" pitch prop your engine will swing (11x4 or close to that).

This assumes you're at sea level of course. Otherwise, I suggest moving to lower ground -- at least until global warming melts the polar ice caps.