It depends on if you've got the RTR or not, I'm assuming you do. You will need a new clutch nut if you have the RTR, (because the crank shaft on the RTR has a unique size and this means the clutch nut included with it is not compatiable with any other engine) and an engine with a threaded shaft, some engines are avaliable with the shaft already cut to the right length. If your not racing the OS CV-RX (S) is a very popular choice. Check out the forums on this site www.rc10gthobby.com
I compiled a list of engines that will work with the RC10GT there (mostly racing engines though). I personally run a Novarossi engine in my GT, which is an SG shaft and requires a custom clutch setup which works very well for me, but I wouldn't recommend doing this clutch conversion to a new guy, just stick with a normal threaded shaft engine.