I have had that exact same problem. but I cleaned mine by backing into a river and let the water clean the clutch. I have been thinking of a way to fix the issue too, but it involves pulling the motor and building a plate to seal the bell housing off. then I am going to put 2, 1.5 inch ports on the top of the bell house for circulating air. I talked to a guy at a clutch shop and he said that the turning of the clutch inside would make its own convection type air flow so a fan shouldn't be needed but if it doesn't work a fan can always be added.
Kevin