Make sure it doesn't have a pin that blocked the rear lug on standard AK magazines. I remember reading where some variants (perhaps NHM) had those, and magazines designed for this setup had a small cutout on the rear lug for that pin. It could be removed to accept standard AK mags.
It should take double-stack AK mags, as I've never seen or recall there ever being a single-stack MAK90.
While popular during the Clinton AWB period, MAK90 quality can be spotty. I've seen where people have had both tight and loose mag problems, but it's usually the latter. The prevailing advice is to try numerous mags (as they also run the gamut in terms of size and fit), and if modifications are necessary, do so to the mags first and only modify the well as a last resort.