Im honestly floored by your experience. The 500 is one of the simplest automotive engines i have ever worked on, and im sure many will agree. Any competent classic mechanic would never refuse to work on a 500, lol. There has got to be a good German/Italian mechanic out there that would be willing to look at it. Classic VW techs could easily work on a 500.
Then agian, most mechanics nowadays are plug and click diagnosis so, "old school" tech is far beyond their abilities.
Are you having engine, or drive-train issues?