This guy sells seal/buffer sets for the mercury and meteor:
I have not tried the seals, so I can not attest to the quality.
A issue that often comes up with old mercurys and airsporters is swelling of the synthetic piston head, that can cause the piston to stick to the chamber walls and reduce power. Oversized O-rings will do the same.
This is described well over at the uk airgunbbs forum.
Knibbs has a replacement piston head, seals and buffers, and is a good source for all things vintage BSA. I gues TR Robb might have piston heads, seals and buffers as well.
The breech seal in these old BSA's can be substituted with a HW seal, I like Maccaris seal, as it is oversize and can be fit to the gun. shimming original seals that are too low is also a possibility, I use dental floss.