Drop the engine. Split the crankcase, leave the cylinders and head assembled to the upper half of crankcase. Save the bearings. no need to break into the top end, this will cost you extra $200 in gaskets alone.
At this point you can take out the gears and replace as needed.
Re-assemble with the old bearings unless you want to change them.

 

Did mine today.Like Paul said you only need to split the cases.
There is a good gearbox with bad bearings and the worn gearbox(2nd and 3rd) that was in the engine.

Transfered the good gears to the old axles that have good bearings.
Notice how small the wear in the dog teeth is on the worn gear.I couldn't even touch the throttle without the gear popping out..

Now,the 12 might be the prince of biking enthusiasts,this bike is the king of cycling!
Handbuilt in the US of A and still rocks!!