the os .18 cvrx is an excellent engine, i run 4 of them, Nitro mt, hpi rs4, Tmaxx and rc10gt, easiest engine to tune, loads of power, will wheelie all of the trucks easily, and you get can for $109 from towerhobbies, and its a direct fit for the hpi firestorm. I think they deliver to the UK too.

http://www2.towerhobbies.com/cgi-bin...&I=LXGGP7&P=ML