Measure the piston from the bottom of the skirt to the top of the hole and then look on ebay, there are some sellers that have both pistons and the one that matches yours.
If you want real performance, don't cheap out. I've tried that and it has always came back and bit me on the rear, and the teeth marks were right on my wallet.
Are those twin SUs or Stromburgs???, Is it British or is it Swedish???...What kind of oil are you using to lube the carbie slides under those screw top caps???, (i used to use Marvel Mystery Oil on my MGs, Triumphs and Jags...That was back in the 70s),...Now is there anyway I can strap that Bad Boy onto my
bike...lol...DAMIEN