Just a couple of points;
Where will you put the engine? The motor is hung out of the back of the the car for a reason, but don't forget that an IC engine weighs more than an electric motor.
Given that it is 2wd where will you manage to fit a brake in? The obvious place is next to the spur gear, but you will already have a really wide rear of the car, an IC engine is about 3" long including the clutch housing.
Where will the exhaust run? With a 90 degree manifold the length of the engine is extended by another 1.5". And where will you run the pipe to, through the rear suspension?
If you want to avoid all these problems then you will have to move the engine to between the wheels. Then you have the problem of how to fit everything in. Plus you will be making the car far more unstable as you are removing all that weight from over the rear wheels.
Good luck if you attempt it, but I think you will find it incrediably difficult.