there are alot of engines that will fit. i have the Rossi pixie .12 in mine. I hadda cut the crank though, as it isnt an SG.
If you want the most reliable, best engine out there, with exception to the super high end mills, nothing can beat the OS cv-r though.
pick up the OS .12 CV-R(SG) it will bolt right in, with no mods. it also offers more power than 2 of the stock AE mills combined! Along with that, u can buy this engine for less than 100.00
There really is no other option, unless you look at the super engines, like rossi, mugen, ect...
I have the OS .15 cv-r in my RC10GT, and it just SCREAMS! plus, the OS brand engines are extremely easy to tune. Good Luck!