Paul,
I don't know what a re-sleeve would cost, but it is probably the easiest option.
If the 250 is anything to go by, the 500 will be a Nikosil.
Options
1. re nikosil the original bore if it's OK & replace the Cagiva Piston (good luck finding one)
2. Re-sleeve & instal a YZ490 piston.
Pictures of the bike haven't turned up yet.
AJ