I put a Castle 1406 system in mine, made the car a lot of fun. Right around the $100 mark. I also got a Venom 2S right before which ate my Firebolt but was fine for the Castle.
Consider getting a spare rear drive belt too after the power upgrade (ran mine for less than a month before snapping the rear belt ... no prob it's a whole $7).
On the road: XV01, XRay T1, HPI Cup Racer, Sakura FGX
On the bench: M04, TA05-S, TT02, SC10, TA03R-S
In the box: TA02-S, M06