cwhite,

The two four strokes are too much engine for this model, IMO. Not from a power standpoint, but size. My current SE has a Saito 56, and it is large enough to make mounting a PITA (and it sticks out quite a bit beyond the cowl cheeks).

This is easily rectified if you're building the kit, as you can move the firewall back. Not with the ARF, though.

I considered the Irvine 53, but now that Sig is no longer the US distributor, availability is somewhat spotty. And I wonder what one would do if the engine required warranty service. So I passed.