Do you know whether Bitmain's firmware checks for unofficial parts at runtime (perhaps like what Apple does) and powers off if there are non-Bitmain parts inside? I think they could use that to force people to buy their own miners.
Modern miners use variable output PSU's that adjust the voltage to between 15-20vdc at well over 150amps.
I wonder if a part like this would work:
https://aliexpress.com/i/4000307508670.html ? This is an external part so I'm not too sure how you'd connect it to the casing.