The rectifier is not likely at fault for your immediate problem but I agree with oldgoat on replacing it for preventative maintenance. Make sure the new one gets a good heatsink- one way or another.
If you're able to find a wiring diagram for this it would help greatly in solving your over-voltage issue but there's no harm done by trying a modest change to the adjustable resistor. Normally, I'd suspect something has failed when output voltage is so far off.
I'd probe the voltages (AC, DC) at that big adjustable resistor tap, move it slightly, measure again and check the output voltage again. Moving the tap away from the hot end should reduce your output voltage, as an educated guess that this resistor is in the excitation circuit path.
Best Wishes,
Bruce