A word I don't think I've ever heard used in American English, only in old British movies, is 'mountebank'. It's close, but not quite identical in meaning, to 'charlatan'.
Charlatan. A person who pretends to special knowledge or skill that he or she does not posses.
Mountebank. A person who deceives others, especially in order to trick them out of their money
'Mountebank' would be better for describing those who possibly possess the knowledge to repair a magneto but who don't waste the time to do so because they know their customers won't look any further than the polished outside surface. However, it wouldn't convey the same feeling of outrage toward a rebuilder as 'charlatan', at least in American English ("Gadzooks, the mountebank miswired my condenser!")