That's a great looking bike; I had the same problem with hairline cracks in the brass overflow pipes on my 10/69 K0, but I didn't have any spare float bowls.
I found I could fix them easily by cleaning the brass overflow tube with a fibreglass pencil, such as this one.http://www.ebay.co.uk/itm/FIBREGLASS-PENCIL-PEN-ABRASIVE-CIRCUIT-CLEANING-FIBRE-/150769690420?pt=UK_BOI_Electrical_Test_Measurement_Equipment_ET&hash=item231a92e734
I then just used conventional flux, a micro gas torch and low melting point modellers solder, it runs straight into the split but doesn't seem to block the pipe...........I did mine over a year ago and they are still fine.......
If you have a persistent overflow and are convinced your fuel level is good and the float valves are not at fault it's well worth looking for these splits, they can be very fine and quite hard to see, but I guess from reading your experience it's probably a common problem.
Enjoying reading about your bike, all interesting stuff.