I assume you have a 4-speed box? If so, there are two possible sources of seepage, 1)Kick start spindle (part 32, Snidal manual) or, 2)Main shaft bearing seal (part 49 Snidal manual). For the first possibility,there should be a thin rubber O-ring around the kick start spindle which may be leaking or for #2, The bearing seal on the main bearing may be missing which would allow leakage. I think the 2003 bullets were supposed to have had these seals from the factory, earlier bullets may not have had them. These seals are available from N-field gear for a very reasonable price.
There is no filter on the gearbox, if there is sludge, just open the bottom drain and let it drain out.
The oil in the gearbox from the factory would be 00-Veedol which is thick an may look like sludge, the factory used this thick oil in an attempt to limit the leaks from bikes without seals.