1. By making the engine sprocket bigger than the shaft sprocket, you will have a reduction in engine revs for any given prop blade length.
2. Make the shaft hollow, devise a way to feed engine oil into it, and you can fit a hydrolic c/s prop.
3. By lifting the thrust line of the prop, you can run a larger, more efficient propeller at reduced revs.
A direct drive auto conversion always scares me, I keep wondering when the prop gyrations are going to finally break the crank or chew up the thrust bearing. Besides, auto engines are don't make enough power at 2700 RPM. I'd go with a redrive just to keep from breaking the crank. Folks have used airboat redrives that have spur gears in order to increase the RPM and raise the prop shaft