Ever since the invention of the Land Rover, people have been trying to put different engines into them, seeking to "improve" upon what the factory engineers deemed sufficient. That tradition continues to this day. Old Series Land Rovers are a doddle to work on, and the rudimentary construction lends itself well to accommodate the ministrations of those backyard engineers who seek to make the Land Rover fast enough to keep up with modern traffic. Though the vehicles are simple enough, the engine bay is small and the gearboxes and axles aren't strong enough for much more powerful engines. So the choices are kind of limited.