Does the 5V STBY LED come on when you press the DIAG LED button? If that LED does not come on, then power supply is likely dead.
However, there will likely be other issues, with both OVTMP CPU LEDs. There may be a problem with the backplane board.
(I bet that the STBY LED comes on when you press the DIAG button )
GET A SERVICE MANUAL. Follow the steps for "Minimum Configuration Testing" - basically taking the logic board/backplane board to almost empty, followed by a few simple steps that can tell you where the problem might be.
I'd avoid and look for another unit, unless you're really looking for a project. You can find fully working used 4,1 & 5,1 machines for under $500 these days if you don't care about the specs.