Yeah, as Baz said first thing is sort the port - open to around 3.2 if it's at std 2.9mm. Possibly more depending on the below.
Weight the piston - how heavy is it ? Also what cal are you ? .177 tend to bounce more than .22
Also what's the stroke ~88mm IIRC ?
You cant just generalise on "light pistons reduce recoil", it depends on all the variables, otherwise all guns would have ally pistons.
You also need to think about what you want from the gun - do you care about cocking effort too ? or Surge ? Or accuracy ? Or full power ? Or hold sensitivity ? Or sight picture movement ? Or just "recoil" ?
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.