I have just spoken to Rich Lewellin (Louigi Moto) as we have the same problem as a few of you with a 99 engine (I think).
Rich can bore the stock barrels out and get the nicosil plated so you can then use your big bore pistons from your kits, that's one way around the problem.
Price will depend on how many of you go down this route (assuming it's OK in the rules)
Drop Rich a call on 01761 453366 or 07775 557797 he's waiting for your calls
Hope that helps