How much mildew are we talking about ?

A little bit a lot or , cant see the PCB ?

One thing you might want to try is putting the PCBs somewhere dry and warmish so the mildew desiccates. You may then be able to blow it off with compressed air (don't inhale the mildew !!!). Then you can look for cracks, bad joints etc. with a flashlight and a magnifying glass.

From that point you can probably right up your local music store and ask to speak to the tech who should, at the very least be able to give you an estimate to have the thing looked at / give you a quote for any work that needs doing.