G3/CETME bbls have to have the chamber flutes to extract reliably, other designs use a plain chamber and would require fluting to work. I heard of one guy who made a rig to scrape/file in the flutes but with a new PTR bbl running just a hundred or so bucks over what another bbl would cost I see no practical need to do so. Gunsmith Murray Urbach was set up at one time to EDM the flutes into bbls, but this process is very expensive and was practical only for use on high dollar match bbls being used for "sniper" rigs. I have heard he may be retired.
True CETME receivers usually have a small area at the rear of the magwell that can interfere with a G3 mag seating, a bit of Dremel work and they fit fine. Most build on a G3 receiver with the CETME sight base. Even a G3 receiver usually needs a bit of fitting in the magwell.
Unfortunately, you are getting into the G3 build game a bit too late. Kits are getting rare and expensive. Here's a few links that may prove useful:
PTR 91 Online Store
G3 Parts - HK91 Parts - HK 91 Parts - HK-91 Parts - HK Parts - Cetme Parts - Original German HK Parts
HK 91 & G3 Question & Answer Page!
Building an HK G3 Clone
Building an HK G3 Clone on a JLD PTR-91 Receiver
There are a few nuances to the G3/CETME build that require attention before welding, the big ones are cocking tube length and bolt gap. No big deal as long as you know what to do before you get too far along,