I used to work in the transit department as a mechanic on E350 buses. Every board we ever had issues with was a nightmare and EXPENSIVE. Those boards were made to spec for the upbuilder and available only from the upbuilder.... if they were even in business by the time a board went bad.
You may be better off getting rid of that if you can figure out how to rewire the things you're keeping. A/C could be as simple as some wires, relays, and switches. The heater should be super simple.