This is a MISR-90, it started life as a semi-auto clone of the Chinese Type 56. Built in Egypt, it was assembled on an Egyptian rec'r and then shipped to the US.
The job here is to replace the original thumbhole stock with a Type 56 stock and pistol grip.
Some have a plate welded in the rear of the rec'r, it has a bead connecting it to the rear trunnion. Knock the plate back and forth, it'll hinge on the weld and will eventually weaken it and the plate will fall loose. Dremel away the remnant of the weld.
Look under the release button and see if there's a hole already there for the front screw for the buttstock. This one didn't have it so I drilled a pilot hole for the shaft of the screw and then countersunk it to make room for the head of the screw.
With the plate out of the way you can test fit the stock.
Jig up a scrap piece of carbon steel and tack it in place.
Weld it to the rec'r, then mark your cut lines. Don't mark the lines exact, leave yourself a lil extra for that final dressing with a file.
Yes, the bead looks like a$$, was in a hurry and too lazy to sharpen my tungsten.
Roughed in with a cutoff wheel, time to take it home and let the Dremel do the rest.
Fairly decent fit, ready for prep and paint.