You are correct about a bad microprocessor, it would need to be replaced with a processor with the proper code on it.
As to how difficult it would be to repair the remainder of the board, that would depend on how experienced you were with electronic repair - and from the questions you have asked, it does not sound like that is your main line of work. Working without a schematic makes things very difficult.
I would start by just checking that all the connectors were making good contact, those can corrode over time, so pulling the connector off and putting it back on will sometimes get things working.