reboring without new piston and JAP 2A ring gap?

hi all,
my little JAP 2A has a poorly bore, its worn to a tapper and badly scored, I have sourced some new piston rings but the best i could do was +10 oversize. so obviously if was to fit them i'd have to file then down to fit ( by the way what is the ring gap meant to be on JAP 2A's???)
since the bore is so worn i will probably get it rebored.
I always thought that if you rebore oversized you need oversized pistons, but speaking to my Mum (who has a VW fastback rebored) she said that she didn't need new pistons just new rings as it wasn't rebored by that much. is this possible with my JAP? surely i'd get 0.010" extra piston slap?

i'll be seeing my local engine machinist this week so he'll answer any answered questions, i just want a bit of knowledge before i go so i don't seem like a complete idiot!

anyway, thanks for any help given,

Callum

problem solved, I found a new piston as as the old one turned out to be past it, the engine is now +60 oversized with a new CC rating of 104CC.

the engine machinist was really good, i gave him the engine two weeks ago, he looked at it and phoned me up two days after to say i'll need a new piston. I dropped off the new piston last saturday and it was ready with a new bore and cut exhaust valve on tuesday. A highly recommended business

Callum