A lot of the parts for the Diana 22 are the same as the Webley Junior rifle, Webley Jaguar and Webley Ranger rifles, some parts are still available for those too but are quite rare so pricey.
I have a Webley Ranger which my daughter shot for a little while until she lost interest but as I had one as a kid too, it's the only rifle I'd never sell unless I found a better example.
I ended up making a PTFE piston seal as the old leather one was well knackered and a spare couldn't be found at the time, works well and produces 5 FPE but accurate as a pcp at 10 yds.
The Ranger is slightly different where it was available with a dovetail for a scope and had an upgraded solid steel rifled barrel where as the Junior and I assume the Jaguar had the same plated smoothbore brass barrel as your Diana has.
Lovely little fun guns and mine has accounted to the extinction of a colony of mice that were too sneaky for traps!
Good luck with it.
Paul