Can a MAK 90 barrel be threaded to accept a muzzle brake or flash hider?

yes.

As in, is it a "normal" diameter barrel as compared to other AKs?

yes. therehas been enough meat to thread every mak90 that i have worked on.

It would be threaded to 14x1 LH correct?

yes.

Is there a need to swap out the FSB like you do on Romanian AKs - to add the locking pin?

no, but you will have to buy the plunger, spring and a cross pin from someone such as k-var. the plunger both times and retains the slant compensator in place.

Being a post 89 AK, but pre AWB - would you have to solder the brake on?

no. but assuming this rifle has a pistol grip, adding the eevil features does mean you will have to have your usa parts in place...5 for a milled receiver with muzzle device, 6 with a stamped receiver.