For either, I would investigate a performance muffler. i.e. ultra thrust or Jett, or even a Tower muffler.
I have Tracer with a 46fx + Jett muffler installed, but not yet flown. If you mount the engine side ways (near 45 degrees) the muffler will fit in under the fuel tank compartment. Just a small intrusion into the compartment - maybe 3/4 inch. It requires building a new hatch cover to accomodate the muffler.
If I find I want more power on mine I'll probably try the 50sx.
There are some other 50+ size engines I've read about here. For example the Irvine 53 sounds interesting. Another one is the the Jett .60L ($$$$)... maybe too much power... if that is possible. It is a light engine - a 60 in 40 case. Or perhaps a YS 63 (4 stroke) - it should permit you to mount the tank over the wing under the canopy. A 10 oz Sullivan round tank will fit. I will try that tank location with the 46fx and a perry pump. ... or or or or ... okay I'll stop
