Any .46 engine. If your buying new then the .55AX is a good choice because it can be used in a number of 60 size planes like the Pulse from Horizon or 4*. You won't over power the air frame of the little Kaos. However, the .55 isn't really enough power for planes like the 60 size Kaos. I have flown the 40 size Kaos with the .46 OS LA and that's all it needed. Any .46 to the .55 is a good choice.