for aluminum parts use thetoyz.com they are same price as the chinese e-bay guys have lower shipping and you get the part in days not weeks (I ordered from both got my parts from thetoyz in 3 days, the chinese ebayers 4 weeks)

and the upgrade for the MT2 a G3.0 engine is $90 on ebay. and the HD final gear set is $25-30 a piece.
to upgrade it it would be about $150 (The older diffs might last for awhile as long as you don't land WOT off jumps) VS $330 for a new one.

Personally I would just upgrade but thats me.