If you have the stock ring and pinion gears you sooner or later will strip them no matter what engine you run unless you drive REAL easy. All so if you up grade the diffs make sure you shim them. You can all ways get a different engine and when the diffs give out fix them then. On my MT 1 Racer I up graded just about everything to MT2 SS specifications other then the radio tray and upper deck that is why I call it a MT 1 ¾. I have run and raced it so much about 90% of the purple is gone on the bottom of the chassis.

You have two ways to go on HD ring and pinion but this is they way I would go as it is the cheapest. You will have to use your old drive cup and set screw. Unless you up grade more.
http://www2.towerhobbies.com/cgi-bin...?&I=LXJYM2&P=Z