Three are lots of ways to approach this. If that Nissan starter is also 1.4kw (amperage, 'horsepower') then you might ask a local auto-electric shop what it would cost to tear down each starter and swap the gears. Or just buy the correct rebuilt starter (or new, replica) from Hoye, or off ebay. I'm not sure you would save much by swapping parts.
On my YM240 (YM2000) someone had already put the wrong starter on it, similar but intended for a later Yanmar-engine Deere. The only practical difference was that the wire from the ignition switch to the solenoid had been extended a few inches to reach the solenoid terminal lower down.
When I eventually replaced that starter I stepped up from 1.4kw to 2.0kw for a couple of dollars extra. I found this on ebay described as
'
NEW STARTER YANMAR ENGINE 3TN78L 171353-77010 AT110818'. It is a perfect fit (after the extended solenoid wire) and works great. The engine spins like mad. It starts instantly and feels like it would even in in a blizzard.
