The 101mm piston gets you about 600cc even. The 11-1 actually will only give around 9.8-10 or so. That's not a bad thing though. Mine has the 102.4 Wiseco and it's only about 10.2 with a smaller chamber XR500 head. The 105mm pistons need the main cases and cylinder both bored and a much larger sleeve installed. It's pretty expensivebut the only way to get over 620 cc. I don't know just who it is that makes these kits but the pices are available. Captain Midnight (on this forum) can refer you to the right company for this mod. I don't believe you could modify stock guides enough to work with the x12 and I'm positve it wouldn't fly with the x35. I estimate my 620 is 48-50hp using all comparable stuff except a stock XL cam. Even the dropin Webcam is good for a couple hp and it's far less money. The big tube headers look cool but don't make much difference in power unless you go big on all the other aspects (big cam, ports, carbs etc). I have the WB S/S header and can hardly tell any difference at all from a stock XL header (which is already bigger than an XR header). The lighter flywheel is a real good idea if you want quicker responce.