Is it for bashing or racing?
If it's for racing then I agree with bobf24 about the OS CVR.
If it's for bashing then get the most powerful engine you can find.
(I use a '03 Fantom FR12R in my Losi XXX-NT and it is one one the quickest trucks on the track because the powerband allows the rear wheels to hook up without spinning. Our fastest racer uses the old OS CV in his GT.)