Piston O ring is a BS212 or if that is too tight a fit, one from a Meteor BS210 will often be fine.
Buffer washer can be made from a 3/4" tap washer or a leather disc; it is not essential to have an original. Breech seal can be made from urethane tubing (or other slightly pliant tubing) 12mm od x 8mm id x c 4mm deep.
If you can make or source these bits you save a lot.
Cheers, Phil