The easiest solution would be to look for a replacement power supply on e.g. eBay. While costly, it's likely to be the most affordable option as well, if there's nobody to help you with repairing the defective unit.
If money is tight, you could consider something like the light source from the Intrepid Enlarger (
https://intrepidcamera.co.uk/products/intrepid-4x5-enlarger-kit)
I'm not sure what kind of enlarger you have, exactly, but it may be possible to fit one of those into the chassis. It'll be far cheaper than the Heiland unit.