I've used after market LGR piston seals from Knibbs & have done "O" ring conversions. Both work just fine but neither as straight forward as replacing a Feinwerbau 600 piston head.

The Knibbs item is too big in the flange area as well as in height in my experience so if you did manage to fit one the cocking lever wouldn't close even with the cam turned to its lowest setting. I have even seen experienced fully trained airgun repairers fall into this trap, the result BADLY BENT COCKING LEVERS This one was sent to me for this very reason.

When replacing the piston seal I always remove the piston from the rod & tap the end of the piston to take a grub screw, then machine a groove in the pin for the grub screw to locate.

I do this irrespective of if I'm fitting an after market seal from Knibbs, doing an "O" ring conversion or in this case MAKING MY OWN PISTON SEAL