I have been running a 6.5 540 and its got plenty of squirt, but im running the sc10 4x4 which is lighter than the losi, if i were you i would try the novak 5.5 540 first and see what its like, then you can tryout my 550 4.5 novak and then just hang on
Also got matching speedo for the novak motor if you want to try that at the weekend