What about a cover for the whole back wheel? If the sprocket is spinning around then the wheel must be as well!
I think this racing thing is all too dangerous. Why don't we bring our bikes to the track, look at them all, maybe start them up (if we can work out a perfectly safe way to do this) and decide who gets the trophies, by reading old magazine comparos. Eg Pre 85 500cc, CR Honda the best on tight tracks, so obviously only those who turned up with a CR can possibly win. If more than one person brought a CR then some more investigation is required. Maybe someone had their front tyre at 25psi, or someone else had an Ohlins shock, etc. A well set up KX may be able to sneak into a place under this scenario.
This shouldn't be too hard and much safer than the current situation of actually racing our bikes. Our bikes wont deteriorate or get dirty. Our fitness wont matter. Young people wont win just because they are better riders.
Cheers, Grahame