[Mr Gazza]

That sounds like too much damage to put right with a re-bore and plate. (Can you get oversize pistons anyway?)

I would have a go at welding some more fins onto the good barrel and keep the same piston in it.

It's not rocket science to shape up some new fins from ally sheet and weld them on.. They only have to be tacked where you can get to.
Or if you're brave you can knock off any fins that are in the way and weld those back on once you've got to the awkward ones.