I know this is an old post, but I figure I would jump in anyway.
I don't recall ever seeing a drum of any kind for this rifle. I have seen people convert M14/M1A mags to work, and I have heard of Saiga 308 mags being converted, but never a magazine. That being said, I would try an X Products drum and see if I could open up the front tab to work on the M90 and trim the locking tab on the back to fit correctly.
If you do this, post pics afterwards, as again I have never seen an M90 with a drum. It would be bad ass though!
X Products DrumX Product Drum In Stock At The Time Of This PostAn alternative drum for less money
M14 ProMag 50 Round DrumJust in case you need more mags, they are OOS as of the time of this writing, but they seem to get them from time to time.
M77 MagsThis guy shows the conversion being made. I don't think I would use a Dremel tool with a wheel to do the front cut vs. drilling the hold open and cleaning it up. I would refinish the new opening afterwards as well. The back tab on the X Products drum is made from the same material as the mag itself, so it should be easy to trim.
Good luck!
Zastava M77, M90 .308 M14 Magazine Conversion
@Stutzmech