I'm thinking gumming up has to do with the grease or oil used, the odd thing is these machines usually were oiled, so the least likely to lave trouble later on. Sometings are down to user errors, but again, grease and wrong oil are the worst offenders.
I use soap, water and microfiber coths for cleaning, and car polish often lift up any last traces of dirt. I guess any staining are oil mess left for 30 years or so, and it usually comes off. If the machine is intact, all parts are there, I'm sure someone are interested in it.
If it's badly treated, worn down, broken parts, impossible to find replacements, etc.; it has to be just the right model to be worth the effort. Don't feel bad about letting a machine go when it's beyond hope, but dirt and grime should not stop you ;- )