Ok I'll try to keep this short and striaght forward
All you'll need is two of the standard Assoc Bevel Gears for there box. You do need to change both because of final drive ratio. Which after the modification chg's from 2.6 to 2.667.
On a diff the assoc gear and balls are slightly narrower. so what you'll want to do is . . . . On the diff half that holds the second inter bearing use a dremel (?) and carefully make the current bearing hold just slightly deeper - not very much. If you don't do this enough you'll not be able to tighten the diff correctly.
On the front one way - all that is needed is to enlarge the hole to just fit on the one way unit. The current ball holes on the assoc gear line up perfectly to screw the gear on. I had made a graphite backing plate and am still using it.
The pinion you'll use is the standard tamiya one.
The last thing is the assoc gear messes better with the pinion and results in excess for and aft play on the drive shaft. I would suggest a spacer that is placed between the bearing on the rear box and the spur gear. This will tend to push the shaft forward to increase the "flat" spot surface area used in the pinion.
Hope this helps
The Assoc gears are $7 US each - the white material ones are stronger than the black material gears. Either work however.