Hell if your going to do this why dont you go for a 600? You could get a CBR/GSXR/R6 motor off a wreched bike for a good price. know that sounds like the ultimate bike. The ability of owning the twisties and the straights:thumbup:The sprockets look like they would be reasonably close since the stock EX uses a 130 tire and the XRR can use a 150 without the chain hitting the tire. IF the motor fit in the frame with little trouble it shouldnt be super hard to pull off since they are both cradle frames. If XRR motors were cheaper I wouldnt consider this but these motors are so much cheaper you could put together an XRR pretty cheap if it fit. I found that the EX500 motor is 115# and the XRR motor is 90, so it is a 25# difference. Then there is fitting a battery somewhere! Maybe too much work if it is just to make a cheap XRR.