Go to CCI or Titus. Mike makes a spring kit for Phantoms.
If you place all stock Phantom internals into your Nellie, it should perform as well as any other Phantom.
If you are searching for that 35 to the "magical" 60 shots above 280 f.p.s. off of a twelvie, you'll need to experiment. There's lots of info on this site about doing this.
But, it seems as though you are just looking to have your Nellie run normally.
Start with all Phantom stuff first. This is where you should start. Then, change one thing at a time and record your findings.
Fearlessly, the idiot faced the crowd, smiling...