That is the type of starter I have on all my 320 trucks, I also had a MG Midget starter that looked like my 320 starters, but I was able to sell that engine so i never test fit that starter to see if it would work on a Datsun engine.
You can see if it will turn the engine over if the engine is not seized, all you have to do is put the transmission in neutral, connect the battery cables up(positive ground), then it would be best to use a remote trigger but all you need to do is give the starter exciter wire post power and it should turn over, and what I mean is power from the negative post on the battery to the exciter post on the solenoid, that will activate the starter.
You cannot just switch to negative ground, the generator will not work and it could possibly hurt it, I do not know if it can switch it to negative ground but someone likely can but why, if going negative ground just use an alternator.
I just went out and tested a starter with a solenoid both ways and it worked both ways(positive and negative ground), it also turned the same way both ways/times so it is likely the starter will work no matter what way it is wired, but the generator doesn't work that way.
If you turn it over and it sounds bad/wrong stop as the teeth on the flywheel might not be the same as the ones on the starter.