There are many "rubber donut sprocket" alternatives! I think it is the least desireable part of the kits.
There is a Grubee motorized bicycle hub that I use on a lot of the wheels I build that has a hub mounted sprocket. I also build disc mount hubbed wheels that can be used in conjunction w/ King's S&S products. I use hd spokes to further strengthen the wheels. There is also a clamshell adapter that clamps onto your hub, I've not used it, but heard good things!
Solid Sprocket & HD Axle Kit w/Brake - $49.99
I believe that hub mounting the sprocket is the single most effective way to make your motorized bicycle more reliable. Good Luck!