The Imp kit is about the only way to go if you can find one.
the problem with converting the PGP itself is the hammer travel.
If you look at any current blow back, the hammer/bolt has a full travel of between 1.25" and 1.5".
The PGP is lucky to squeak out .375".
You'd also need to convert to some form of floating sear.
I messed with it several years ago, here's what I did to make it work....sorta.
1) extend the hammer tube 3"
2) stainless hammer with a .5" long snout necked down to 3/8"
3) delrin bushing:.5" by .77"od and .502"id. this part fits against the valve and allows the hammer to have a sealed blowback chamber.
4) delrin bolt, pinned to the hammer at the rear
5) heavily modified Spyder trigger frame.
6) file the valve pin flat to allow blow back gasses.
It worked, but not very reliably and I had too much to do to spend time on it. I think it's still around here somewhere.......
Have a great day!
Lee Kinney aka Punisher
"Because the Dead-Box is a lonely place."