Which woofer would fit best?
Store bought preferred.
The right one is getting scratchy

Have you tried tuner lube spray on the controls including the balance first? if not a little de-oxit spray brand may help relieve the symptom

the m90's woofers have an over sized mounting flange ,this means you will have to modify the new woofers to bolt into the m90 as standard 8" woofers mounting holes will not line up unless you bend the outer speaker frame flat and enlarge the mounting holes.