I haven't done much with engines smaller than a .29 but in the range you've mentioned I don't think you could do much better than an Enya 35. They last forever and give good power for their size. The 35 is better than the 29 for general flying because the 29 was designed for somewhat higher power so not quite as useful. They come up regularly on the bay, in fact I picked one up not all that long ago so my other one wouldn't feel lonely . Another great engine is the Merco 35, especially the earliest versions, but they're a bit rarer.