I had a full metal piston way back in the day, I believe it may have been Lonex or possibly Guarder. I do know Lonex makes them. along with piston heads and so forth - actually very reasonably priced. I wouldn't necessarily bother with a bore-up kit, just change to a full cylinder.
One point to mention, the Lonex is definitely aluminium and very light so it may not be what you're looking for.
@Lightf4ng - are you trying to get as much power as possible out of your setup? 1Tonne or Cripplegunner have more experience with AEGs over 2.3j, mine ends there.