I have looked into dropping a 748 engine in my S2R800. I can be done, however it would be much, much cheaper to buy a new street figher...seriously, it would take mega $$$$ to get it in and get it to work right. That being said, for the cost of a 4 valve motor you could pick up a used 1000cc 2 valve, do the heads, pistons, crank, clutch and flywheel and have a really mean 2 valve under you. This will be my next project.
i have heard through the grapevine that there is a frame crossmember in the way on 2 valve bikes that doesnt allow the 4v verticle cylinder to fit without modification
obviously the heads on the 4v are huge in comparison
maybe said experts will confirm or deny this