The X's longer swing arm is worth fitting - I'd rate it as the single biggest difference between the way the two bikes ride. You want to X's stiffer shock spring to go with it. Can't recall if the shock is longer or not, but its definitely the same basic unit between the two bikes.
The other info is correct (porting, pipe stinger, carb, colour, forks/triples, muffler, ignition, ignition cover, chain/sprockets, rear hub). For the sake of accuaracy, the C has wheel rims with the external lip, while the X has straight sided rims.
I've not ridden with a stock X barrell, but apparrently they're quite peaky - fine for grass track, but a bit difficult otherwise. Then again, the 125F is supposed to be peakier still - so if you can handle the F, I doubt that the X will present any dramas.
My X has a ported C barrel on it, and its magic - usable bottom end, strong tractable midrange and reasonable top end, although its got no over-rev at all - rev it too hard and its like you've pulled the clutch in, hit the killswitch and ridden into thick goopy mud all at once... The peripherals seem to check out, so I'm figuring that its just a trait of the porting/pipe/etc.
Yes, I've up to my eyeballs in building the rally car, and now cleaning up my pig-sty of a yard (because The Boy is mobile and love to be outside... Wonder where he got that from?
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