Im looking into buying a spare engine from a XT660Z and doing a rebuild of the engine and while the engine is apart upgrade it for some extra hp.

Ive seen that some people say that the stock barrel can be bored to 103mm but not more, but at the same time there are 104mm pistons on the market. Is it possible to bore and replate a stock cylinder to 104mm?

If you would like to do a proper build on a XT 660 engine, what would the modifications be?

Im thinking of bore and replating to 104mm piston, ported cylinder head, OTR cam, exhaust with better flow and improved airbox. What else is there that gives a bit more punch?

If i would do the things listed above, how would it affect the durability of the engine? How would the engine run compared to a 690? Would it just be a waste of money since im better off with a 690 from the begining?