If I can jump in this tread and answer your question, it depends upon where your leak is coming from. If the leak is from the un-sealed 6303, then all you need is a 6303 ZZ which includes the seals for that bearing. However, if your leak is from around the kick starter shaft, you will need the O-ring that fits around the kick start shaft, .......and if your leak is coming from the final drive shaft between the spocket and the gearbox, you need to know if the final drive shaft bearing has the seals in place. If not, you would need those seals, as well as a small O-ring on the sprocket side of the final drive shaft.
If I were to do this again, I would also add an oil thrower ring (thin washer) on the inside of the final drive shaft. I don't believe they install these thrower rings on the modern gearbox but I have seen some old diagrams from the 1950's (before oil seals were common) that show an oil thrower on both ends of the main shaft.