I would say drop in a MM 5700 or 6900 system. Both will be fast... ;) They have been going for under $200 lately even my LHS had one for a stupid cheap $179 a couple weeks ago.
But even at $212 it's the best deal going....
Jammin CRT MM/Neu 1515 1700kv
Losi 8IGHT MM/Neu 1512 1900kv
Kyosho 777
T4 MM 5700
B4 LRP
XX4 MM 7700
old losi xxcr, MM4600 4s lipo 70mph+
i would n't expect too much from it. i had a stock one and managed to strip a front differential plus its belt drive with no way of tightening tension. i think a brushless would destroy the drive train in one of those.