Vintage John Deere riding lawn 1987 285 needs a hydrostatic transmission replacement

Does anyone know what I could do to replace my very old 1987 John Deere tuff torq hydrostatic transmission. Everything else on the mower works fine. This tranny lost power and will not move the mower. the filter and fluid have been changed.

Thanks in advance!

Vintage John Deere riding lawn 1987 285 needs a hydrostatic transmission replacement