Hi...if it is a gas engine, it most likely is 6 volt, and positive ground, given that's what my Olivers are....meanwhile,starters are generaly wound for direction, not polarity, so it should turn the correct way no matter.
The wrong polarity may damage your charging system,however, but if your just trying to spin it over, then disconnect the generator whilst your monkeying with it.
You can also put the twelve volts right to it for short stints, so long as you don't overheat it enuf to bake the varnish on the windings, you'll be fine. I switched my tractor from 6V-Pos to 12V Neg 3 years ago with narry a peep. I did, of course, have to change the generator, but the starter remains to this day. It does help that it fires right up without long crankings...Good luck