I watched @53gunner weld one back together years ago. I think he used all the original receiver pieces. It was mostly a Yugo M53 kit.
If he obtained any new replacement sections, they would've come from
BRP but I can't remember.
You'll need to make or buy a jig to hold all the receiver sections in alignment (the full length of the receiver) as you weld, and you'll need to TIG.
It's very easy to let it warp if you aren't careful, so tack everything a little bit at a time, then go around alternating with you fill welds.
In the end you'll be grinding, polishing, deburring, and truing it up everywhere to get the bolt to slide smoothly. The better your alignment is during the welding, the less work that'll be.