Chain........98% efficient but needs oil unless you use o or x ring, then this needs some oil occasionally for the rollers and to keep the o or x ring seals from drying out and then only appx 94% efficient
Belt....expensive not as efficient in its transmission of power but clean and quiet.
shaft...no where near as efficient as the others especially if there are straight cut spur or bevel gears, needs hypoid oil ..expensive... but quiet and very low maintenance. does get hot hence the lack of efficiency.
if you use chain you will probably need a 3/4" chain for x or o ring chain no 630...steel sprockets all quite heavy as a package, this will in turn generate noise unless tensioned correctly preferably with am automatic tensioner, but it will give you some extra power at the rear wheel, use the same ratio as the htd pulleys as on the vehicle now.
I would leave it as it is, and I for my sins am the uk engineering director of the 3rd largest chain manufacturer in the world
