An engine replacement works great, because a car's low weight and sport-style brakes and suspension would mean that even a slightly bigger engine would greatly improve its performance. This isn't the case with every car. You need some base-line performance to make the upgrade worth it.
However, an engine swap, like a kit-car build, isn't an easy process. If you need to reinforce the frame, welding may be involved. Removing the old engine and putting in the new one requires a lot of planning and effort.