Start here for a proven stock adapter, then look thru their other options.
Yugoslavian Slant M4 Fixed Stock Adapter - CANIS Design Group
Only so far you can go with your original.

 

Hello everyone, I was just wondering if anyone could help me. I have this Zastava M77 with wooden furniture, but I don't really like it and I would prefer if it looked like the one on the bottom picture. I can't find it anywhere online. Does anyone have any ideas where to buy it or does anyone have it at home for sale? Thank you very much Richard View attachment 49786

I have one with the plastic furniture and I would love to exchange for the wood furniture