Hi Freddy, a big welcome to the GTA forum and family. I can't really answer your question as I've never done anything like that to make my airguns more powerful....other than putting in a abit more stronger spring.
Up in Canada only <500fps airguns are sold easily to the public...any over that speed requires courses and paperwork. So some airgun brands have different markings on their models for under or over 500fps airguns. The Crosman Phantom and Quest full power models are about 3" longer....their receivers are longer to contain the longer/more powerful spring.
I've checked the specs for the Ruger Air Hawk.....and the lower and full power models are the same length. So I'm thinking a few modifications with the powerplant...longer and thicker spring, shorter back spring guide etc. might make the less powerful version the same as the 1000fps one. Someone here have also mentioned that sometimes the less powerful versions of airguns might have a hole drilled in the piston head to let air escape. He have welded that hole shut and the power increased.