Restoring a car is a process. You would be best off buying a "how to" book on basic restoration.
You must decide at the outset, whether it's going to be a true "restoratin" (all back "original") or a "restification", where certain changes and modifications are done. Once a path is "chosen", STAY ON IT.
Parts for the '81 are readily available. Ames Performance and Performance Years are, IMO, the two best places to get Pontiac resto parts. They don't sell Chevy or other GM stuff, Pontiac ONLY.
The engne is another matter. Very little true "expertise" out there. There's a 301 forum. I don't remember the name. We've had success with them using parts from "other" engines and custom-ground cams (sounds expensive, it's not). The turbo is quite unique. ONLY a qualified turbo shop should TOUCH it.
Very "special" cars, IMO. They represented a "last gasp" of performance from the old-style cars. Not until the late '80s did performance approach the same level in T/A. 305 just didn't cut it...