I'd opt for the TT75FS to replace a 46-52 sized 2-stroke.

Why buy a .52/54 4-stroke when, for just a hand-full of pocket change, you can get an engine that has more displacement and more power but no more weight.

If you're worried about "overpowering" your plane just remember that you can always throttle back :-)

As to fuel economy most 4-strokes are more fuel-efficient than comparable 2-strokes except perhaps the Saitos. Even when you tune them properly (which lots of folks don't) the Saitos seem to drink about the same amount of fuel as the equivalent 2-stroke. I'm not sure why that is but it may be related to their relatively low compression ratios (which is why they also *love* 30% nitro).

The TT 4-stroke engines seem *very* fuel efficient. My P51 Mustang (with TT91FS) had a 330ml tank in it and I'd get totally bored while flying it *long* before the tank got empty. In fact I replaced the tank with a 250cc one and I still get flight times of 10+ minutes, mostly at full throttle.