[AmiSMB]

The rear part of the centre diff mounting is usually replace with a CNC aluminium part which holds the motor and the pinion it the directly agains the spur gear for direct drive.
The CNC part usually places the motor on the opposite side of where the nitro engine goes. Then with the engine and fuel tank gone the speed controller and battery are then put in their place.
There are some newer layouts where is you are running individual 2s lipo packs you run one pack either side to keep the centre of gravity lower and balance the car more.
I currently use the Tekno RC conversion on my Losi 8B and this has the option of using a clutch or direct drive and mechanical brakes. One benefit of the mechanical brakes on the Tekno RC conversion is that it removes the braking from the electric motor and reduces the temperature and you can still alter the front to rear braking bias.
Some aluminium conversion plates state you can use mechanical brakes but from what I have seen is that they only allow one disc to be used and it is usually the front one. Obviously the down side of mechanical brakes is that you need to run an extra servo and need a transmitter capable of channel mixing for it to work although you could probably get an electronic mixer from a model shop that does aircarft to do the same job.