Contact Gallen Krueger support and see what they say. They've probably seen this many times. Hard to say if they have parts. If they don't, not many people will.
I had that head, should have kept it. I ran it in bi-amp mode into a Peavey cabinet that had 1-15" and 2-8"s. The 15" was replaced with an EVM 15.
It was a great amp.
So it comes down to - do you need it? Do you want it?
Even if you can get a working head for less, one that's been serviced at the factory is going to last a good long time.