I will lead with the caveat that I never put my CETMEs together, lost interest in them and sold the parts kits. There are two folded receivers sitting on the shelf though, and a MP5 flat waiting for me to get to it. That said..... its not rocket science. A TIG welder is not required. If you aren't a complete hack with a MIG, it will work. There isn't that much welding to do anyways. With the exception of the barrel trunnion, most isn't really structural/critical. Most critical welds are on the cocking tube, but thats just so you dont burn through and end up spending forever trying to get the slag out of the inside. Heat sinks are your friend there!! Seen them turned from copper and aluminum rod, and even seen heavy copper pipe put in there and work well.
Critical dimensions on bending the flat are the height between the rails and top of the receiver. There are guys who have bolted a piece of pipe down, clamping their flat to the bench with the pipe in the middle, and "bench bent" it with their hands and screwdrivers. Not the most precise way, and IIRC they usually tweaked the receivers after to get them to work.... but the guns ran. You can do it with basically no effort too, if you want to get real creative as the owner of another forum did. (Mike928, has a thread about it on the MilitaryFirearms forum) Cut a "window" out of what would be the top portion of the receiver, like a 1/2in wide slot, leaving an inch or so on either end that would get formed around a rod. (20mm was the rod diameter IIRC, or a close standard equivalent... the dimension matters to some extent) That window was later "filled" with a channel that was welded on, basically creating a flat top receiver.
Really Im talking out my butt though GI1 since I got bored with mine before I finished them, but the gist is don't get scared or feel like its an insurmountable task. LOTS of guys have gotten them running without fancy tooling, though there are some tricks to making them run happy. Things like setting the height relationship between the lower and receiver, cocking tube gap, clip and pin on the trigger pack, etc.
Theres a member who joined here not long ago, he is THE MAN with these, at least IMO. I suppose I'll "out him" though he may have tired of the platform by now GI1 Holescreek. If you google "Holescreek CETME" you'll find pics, videos, and tutorial posts on forums all over the internet. Last build I followed of his was a gas operated CETME in .243 COOL! The info is out there.