Using lockdown time to go through all my airguns not working properly.

Got to my errant Remington Thunderjet, bought this to have a cheap fiddle with gas rams and got what I wanted.

The gun is weak despite me taking it apart for a good clean up and relube. The piston seal is a bit tatty looking but can't find a replacement.

The ram itself is 300mm in length with a diameter of 19mm. Gone through my old springs and found a Weihrauch one that is 19mm o/d and about 255 length.

This has worked with 14mm of slip washers in the piston end, the spring sits on top of the trigger block as its too big to slip into the block as the ram does.

It cocks and fire smoothly, with the ram it was at 645 fps average/7.67 ftlbs with superdomes (.177), with the spring and absolutely minimal preload its at 735 fps/10ftlbs

So, can anybody recommend a spring? needs to be 19 o/d and 300mm or more? hopefully the 19 will fit inside the trigger block and not rest on the top, will make a guide for it when I can borrow a lathe.