Wow thats a strang brake set up I have never seen the disk on the front of the center diff? Anyway here is a link to some great info for your conversion that i think BrianG was refering to.
http://scriptasylum.com/rc_speed/
Check out the motor selector if you dont know your diff ratios just use the preset 1/8th scale buggy ratio's there's also alot of other usefull info there.
I dont think your going to find a bolt in mount anywhere for that car making one is not to hard as long as you can cut the slots for the motor if you dont feel comfortable doing this take your center diff and motor with a picture of a brushless mount or car with a brushless conversion to a local machine shop and they can fab one up for you.
7.2v to 14.4v is a big difference to find a motor that will work well on both voltages? it's basicly going to be 1/2 as fast i guess you might fry a single 6 cell pack?
Looks like you already have a plastic spur? if so your lucky thats been a problem area with some conversions. :)