where abouts is the oil leaking from, is it coming from within the box (remove the flywheel inspection plate and look for oil inside) or is it on the outside of the box?
if its on the outside then there is a liquid gasket, you effectively need to strip the box down and remove the old gasket, then apply new liquid gasket to the same area. Don't be coy with the amounts of gasket sealer, apply loads as any gaps will allow oil to escape again.
Either way the box has to come of and i would also replace the clutch and shaft seal.
if i was in your shoes i would just top the box up regularly until the clutch needs to be changed, then hit it all in one go. Major job and not for the feint hearted tbh