I am repairing more and more HP41 calculators lately. I have a backlog of more or less one month - and calculators keep coming. We offer this as a service but do not see it as a business, since it is not "scalable": I have only so many hours, and they cannot be increased - except at the expense of more important things to me.
My opinion is that most of the repairs could be done by the user himself, and only the very special cases be sent to us. Really 70% of repairs do not require specific skills or unavailable parts. Then 20% may require screen or processors that are not available - and 10% cannot be repaired at all.
We have added a couple of pages to our book "HP41 Repair", which you can find in Amazon - we really think with it you can repair most of the calculators.
One think that is true is that more and more the plastic of the hp41 calculators is becoming brittle and more difficult to repair. The material does not seem to age well, and lose the plasticity it once had.