Not that I've looked, but I would have thought someone would make a frame consisting of a petrol engine and an alternator, specifically for this sort of task. Something like an older Lucus alternator (with embedded controller, rather than a modern one controlled by the engine management unit) would probably work well. Even on my decades old mini, that could output 45A peak (and 32A on tickover), and there were probably larger ones for larger cars. It contains all the smarts for charging regulation, and doesn't care about clean power input (which is mechanicanal in this case anyway).
They have inverter outputs, designed to generate just a very slightly fractionally higher voltage than mains, so they feed back into it the rate they're generating. You can't use them without having a working mains supply - they're deliberately designed to switch-off if the mains goes away. Apart from having no voltage to regulate against, you don't want something back-feeding a dead supply network and electrocuting someone who might be working on it.