engine at the back or at the front
Hello your question seems a little vague??? but if your asking whether engines can be mounted in the front or the rear of a vehicle then yes to both, and some vehicles have mid engines where the engine is in the middle of the vehicle.
I have seen an Audi TT with two engines, one in the back and one in the front. But i don't think carmakers build chassis that can receive the engine at two different places.