i was trying to do the same thing a while back, i think rc monster has a few different center diff bulkheads that have a motor mount machined into them. they dont have one specifically for the hell fire but if you search my previous posts one of the other members posted a link to the one that would work.

there is always making your own.

as far as pinions, rcmonster.com also has pinions the same pitch as the nitro stuff but make sure you get the hardened ones because the narrow spur on the hellfire will chew up soft aluminum ones