One thing I would say is that be careful if you change the piston ring. They are easy enough to take off but since they are so thin & hard they are quite fragile & so need to be put on carefully as they can't tolerate much movement & can snap if not done correctly.
I think it was wonky donky, or someone similarly knowledgeable about FWB springers, who said that in all his years of experience, he'd never come across a piston ring that was actually worn enough to need changing.
Vintage Airguns Gallery
..Above link posted with permission from Gareth W-B In British slang an anorak is a person who has a very strong interest in niche subjects.