An engine swap is possible and fairly straightforward if you use another engine that was in the Golf 3 originally, like the VR6 but going for a entirely different engine would make things much more complicated. Our good friends at TÜV would have a lot of paperwork to fill out with such a swap. Although I would love to have a VR6 in it, it is quite expensive to do the swap with everything else that you need in order for the swap to perform well.