Is there any indication of what happens next if you do not comply? If not, have you inquired to see what the penalty may be? How and where are you to respond or contest the demand? I suspect that this probably isn't the best place to get advice since I don't believe any of us are attornies practicing in CA so probably you need to ask some questions to see what will happen if you don't relicense the vehicle. Depending on what the response may be, you probably need to either leave the state of CA for a period of time or contact a local attorney.
Is there a craigslist (or Let Go or Facebook Marketplace) for your area? If so, old tools are often listed for sale. When I sold my house, my realtor listed a bunch of Craftsman (and other stuff) for me and, almost all of it sold.
I like the suggestion re: new motor or, new brushes for you old motor.
See if you are able to locate a local mobile RV tech as that would probably cost less. And compare the expense to the risk. How confident are you in your own abilities to check it out yourself? If you send me your email address I can share a checklist with you, should you choose that route.