I had read about a fellow who built one with a 700 HP Falconer V-12. Of course, the airframe had to be beefed up quite a bit. Also, there's a company in the states called Thunder Mustangs that builds an 80% P51-D with the Falconer V-12. Considering the real deal can cost close to a half mill US, these replicas are probably as close as the average (read well-off) warbird enthusiast will get to flying their dream plane.