Oh I'm sad to hear your board is dirty. :/
Well, you can restore them pretty good if the contact leaf springs not rusted. I had a Tai-Hao APC-8 PCB with clicky Apls clones in it. I harvested the switches, took apart all of them, cleaned the parts separately, and now they pretty nice. Take special care to clean the sliders, and the contacts.
If the keyfeel doesn't improve by cleaning the sliders, contacts and housings, you should consider lubing the switches, it helps a lot.
If the board is in a really bad condition, you should consider harvasting the switches, but you can clean the rust off the mounting plate, using a sandpaper. Unfortunately, you have to desolder all the switches to make you able work with the mounting plate. If the keyboard works nice, it would worth putting it together again, after you did the restoration job, but if that not the case, save just the switches, keycaps, satbilizers etc. and you can bulid a nice keyboard later.