These rifles give less than 10 ftlbs in .177 and about 10.5 in .22
Over-springing the rifle to get more power usually makes the gun inaccurate and nasty to shoot
Just use the standard spring for the job
The piston heads are normally aluminium on later models and nylon on earlier models
If yours is metal but not aluminium then it is probably an after-market steel one that can be purchased to up the power by increasing the weight of the piston. I have not found them to make much of an improvement.
Sizing the O-ring can save a lot of running in time and help get the power around 10.5 ftlbs in .22
Do not be tempted to put in a square section spring, the gun will not appreciate it
There are no seals in the loading tap and these taps do leak a bit, but there is little you can do about that.
Good luck with the rebuild
ATB
John