Just a thought. 20 years old and 400 hours means it has been used for an average of 20 hours per year.
If the engine runs ok, then how about trouble shooting the power unit yourself? Lack of use may be the prime problem.
If you are game to learn something new, there are members of this forum that can help you with the process and lots of instructions on the internet.
I experienced that support several years ago when I had to diagnose and repair a Printed Circuit Board (PCB) in our MH. There were no repair services, there were no substitute products being sold, and the manufacturer was not in business.
That board still works fine in spite of my absence of tech knowledge or skill.
Lots of skills are found on this forum to help members.