I am not totally sure about the motor mount thing. You can call and ask RC Monster Mike and see what he says.
I completely forgot that your car has a two speed. I have seen a couple of different ways to go about converting. 1. Ditch the two speed since electrics really don't need it. 2. Convert a 2 speed clutch bell to work on an electric motor. I know Fine Design converted one of these cars, but their conversion was a bit ridiculous. They did use the 2-speed with a modified nitro clutch bell. I can't say I recommend buying anything from them, but you can look at how they did things to get an idea of what you might want to do. You can see what they did in this picture
They did some other things that are just alittle crazy like water cooling, a radiator, and separate power source for the cooling.
The method of stopping your car can go either way. Some people use the motor for braking and others use the disk brakes. I personally use the motor. It will cause some extra heating in the controller, but the new controllers seem to be built for the abuse. The thing you lose with the motor brakes is your brake bias (front to rear). This doesn't bother me.
Does your car have a center diff inside that two speed? If you decided not to use your 2-speed and you want to use the motor for braking you will have to put in something else. The two speed has a 1-way bearing in it which will defeat the motor brakes.