I think the people here who are posting essentially to keep things as they are have really underestimated the loss of knowledge that can't be replaced. It isn't a matter of whether the old parts should be sold, that is the business they are in. The questions are whether they can connect the product to the people who will pay what the parts are worth, and whether the store is going to be able to resupply after the parts are sold. I imagine the old man's knowledge of sourcing was encyclopedic, and just hope they can figure it out from records, friends of their father, etc.
If you're going to be the store that has the parts no one else does, you have to find supplies of those parts. If they can't do that, they may have to become a different sort of store if they want to stay in business.