You need to get the mount first, as the height of the center diff will play a huge role in what fits. If the diff is dead center in the hole it will be at it's widest point, moving the gear up or down from there could give you enough room to make one fit.
Rc-Monster Mike makes custom spools. I run a center spool in my LST conversion. A spool with a traxxas gear may be the only thing you get to fit in between the chassis rails.
Even the smallest center diff spur I know of (44t) will take some trimming unless you can get it's widest point above the chassis or to land in the gap of the center braces.
2 other things- A gorillamaxx single speed converted tranny with the FLM half case would probably fit the hole.
Here is another manufacturers alloy e-maxx tranny case, maybe it would fit and you could deanodize it? >
http://www.ebay.com/itm/Alu-Gearbox-...item2c5eeec328