Im using this one now http://www.hobbypartz.com/thac6smbachw.html and it works great.
As far as mods go it depends on what you are doing. Are you just bashing or are you racing?
Racing definitely needs new wheels and tires. Bowties or crimefighters work good on the HF especially if you put them on 1/2 offset wheels.
A stronger steering servo makes a big difference too. If you want more speed and torque go with an LRP ZR.30 truggy bump start engine.