yeah I'm thinking the air ram from a modern gun modified to fit would be ideal.  wouldnt be worth the effort to cut the ram open, find seals and weld it back up then re gas again. (If that's the process). 

Im a toolmaker by trade and work in an engineering shop so can make things if neccessary. 

 

With the "air springs" or gas rams they are all sealed units. you can not do much with them. You can get them re-gassed for around 40 bucks but if it is that old the seals are probably gone anyway.

And as Paul said the piston seal is pretty easy to make.

I would strip the gun down and get the sizes of the gas ram and also note how long the stroke is on the piston and we can go from there. you can get replacement gas rams from other guns that will probably fit that but will need to know sizes and the stroke first.

It's great you have your childhood gun. Mine is a Slavia that I still have.

Holy hecka, had this about 30yrs not knowing what it is.  Only just now found it's a Relum Tornado after posting here. Might actually be able to get parts for it.

https://www.shootinguk.co.uk/reviews/relum-tornado/

Hey,

So my childhood air rifle has seen better days. Shot alot of pests in it's time. Has lost so much power now it's no longer useful.  If I remember correct it was a Kelly brand? An under lever .22 with a flip barrel to load a single shot. Bought it from a guy in Glenbrook beach who traded guns.  He'd retrofitted a gas spring. Said he could recharge the cylinder if required.  but that was in the 90's.  Is anyone doing that sort of service these days or do I look to find a modern equivalent and make it fit??

I'm a toolmaker / engineer so I enjoy modifying.  I emailed Youngs and they said the air springs they sell arent serviceable. 

Also the piston has a leather seal which needs replacing. Has sucked a bunch of slugs into the chamber which then blocked the port but kind of messed up the seal. Any help appreciated.  I'll get some more pics up if theres interest