Welcome to the board!
On the switch thing, Grayhill are in short supply everywhere right now due to the pandemic. The key question (and I'm not familiar with this project) is whether the switches are PCB mount, or solder tag.
If they're solder tag, then you should be able to substitute something else such as a Lorlin / Blore Edwards etc, as long as there's physically space in the front panel layout and you choose the right contact form (shorting / non-shorting). You'd need to take account of other factors in your front panel design and knob selection - that might include bushing diameter and length, shaft diameter and length, and such.
If the switches are PCB mount then I'm not aware of any other switches that are the same pin spacing as Grayhill. You might be able to get round it by wiring up a solder-tag switch off board, but that might compromise something mechanical (again, I'm not familiar with this project so you'd need to take a view on that).
Hope that helps!