At a guess, it's knackered it's big-end (and maybe little-end) bearings having run with no oil. In that respect, it's going to be exactly the same as the petrol engines you're used to. I'm not familiar with the PH1 (the only Petter I've ever owned was a PAZ1), but there should be some easy way to access the crank bearing. The little end will presumably require the barrel to be removed to extract the piston out of the top. You'll also want to check the rings & bore for excessive wear. So far, all the same.
The main difference with a diesel will be bleeding the fuel system after you've replaced the bearings: Any air bubbles in the system will stop it running.... But I'd recommend doing all the bearings first. I'm guessing that the "down on power" is due to excess friction due to buggered bearings. Assuming it didn't stop itself due to lack of oil, it's unlikely you've terminally damaged it, I suspect.
WRT ring gears: I added a flex plate from an old automatic engine to the generator head on my Lister CS setup. The PH has a smaller flywheel though, so I'd say drill & tap the flywheel to accept a flex plate (there should be a billion of them in any nearby scrap yard), then figure out a way to hold a 12v starter motor in the correct place. However.... that's based on not knowing how your dumper connects to the engine: If it's got a bell housing that covers the flywheel, then you might need to do something different... There may even be a kit for your specific dumper/engine setup?
