Standard Magneto can supply the magneto parts for a Bosch I'm pretty sure. Just got a bunch of Wico parts from them, all new

In D8Dude's photo, there's a spring between the two latches, at those adjusting screws. So squeeze there and the latches will unlatch. If it was latched, the pinion will spring forcefully out towards you, so use care

The pony crank has seals on both ends. So if what you poured into the pony crankcase also came out of the pinion gearbox, then the inner end seal has also failed. The good news is ponies are extremely easy to rebuild. Most of what you need to do is turn the main journals, and procure new bearings and seals and dowels. If you want to go the extra mile, you can do what I and a couple of others have done and that is to fit thrust washers on to the pony crank to take axial thrust. But it's not necessary. Get the gasket set from Cat and you're good to go. They really are simple engines. For your main engine, was there any water evident in the drained crankcase oil? If not, is there any evidence of water having gotten into the cylinders? Also, is the output shaft from the engine (or better, what it's connected to) seemingly free to move / backlash? This is to determine whether it's the main engine crank is loose or something else is hanging up. If it's just the pistons/rings that are stuck, you can free them up either with some leverage on the flywheel ring gear or you could pull the cylinder head and light some fires in the bores to expand them and then turn it over. There's a bunch of techniques yet that you can use to free it up.