Hi, I have been reading the forum for some time but this is my first post...
The method I used to fit a sport tuned motor to my Sand Scorcher was to alter the motor to fit the gearbox lugs. To achieve this, I removed the material between two of the cooling slots to form one long slot, this allows the motor to engage the gearbox lugs. I used a re-re motor cover which I had to dress away a little on the inside to get the bolt holes to align with the gearbox, this was due to the terminals being in a slightly different position (you may be able to remove the wires and bend the terminals, I didn't try).
I removed the sticker from the motor and gave the case a rub with emery to remove high spots. Additionally, I dressed the gearbox housing a little to allow the motor to fit without being too tight. The gearbox is an early example with two lugs to mount the rear cage.