Just an update, in case anyone else is in the same quandary as me.
I never heard back from Megadyne, but after a bit more digging, I found that
SKF Power Transmission do the right size belts (code PHG 390-XL-037 for the longer belt), and
In Stock Direct sell them online.
I've ordered what I need, I'll keep everyone updated when they arrive! In the meantime, back to soldering up my hobbycnc board
