The backplane for the NV, NV+ (V1), NVX, Ultra4, Ultra4Plus, and Pro4 are interchangeable, but not available separately. If you pick up a non-working unit that's got a different problem (bad power supply, for example), you could use its backplane. You could, of course, purchase a working machine for parts, but they tend to be listed for more than I think they are worth.
But, frankly, you are better off buying a new NAS and selling yours for parts. Having a working power supply, I think it'll sell.
Some time ago, I picked up a used Ultra4 with a similar issue on two bays. It had been dropped. I created a working hybrid with an NVX (which cannot be converted to OS6) and the Ultra4 main board, and then could update to OS6.
