Bruce, the gasket should be readily available from many of the sponsors above or your local Cub Cadet dealer. There should be a fill plug on the rear plate.
Here's a link to the FAQ that has info on changing the fluid in the rear end. This is written mainly for a hydrostatic but excepting the filter info it should be nearly the same procedure.
Here's a link to the FAQ that has info on changing the fluid in the rear end. This is written mainly for a hydrostatic but excepting the filter info it should be nearly the same procedure.