Spring replacement in spring airsoft?
 

So I'm restoring my spring airsoft and need to change the piston spring. I was thinking of just buying a spring in the home depot. I find a stiffer one that is the same size as the old one, can it make my gun more powerful?

why?

Precisely.

For the cost of buying a new spring and oring set (is this the same pistol you nuked with WD-40?) you could easily direct those funds to saving up for an actually decent pistol or even rifle.

There's no practical point in restoring a springer. You'd be better off using that money elsewhere.