I just use different size pulleys and a belt when I want to slow down a motor. Might not be applicable to your situation.
My tow-behind leaf sweeper is powered by the wheels and is not very useful to me, only slightly better than raking by hand really. If there's no wind a leaf-blower works better. :(
Eventually I'll solve the problem, but leaf season's only once a year, so I haven't managed it yet. One thing is clear: I need a really titanically huge leaf container, or a way to shred the leaves as I gather them, or both. My property is at the low point of a heavily wooded stream valley and the leaves pile up several feet deep against the buildings and other obstructions. I have to empty the tow behind sweeper every two minutes or so.