What do I do with the breather ? I seem to recall reading that there is an optimum length for the breather pipe. How long should it be ?

What to do with it - leave the end open, and dangling outside the fuz. How long, it should be in the handbook, or look on the mfr's website. Being a glider/electric man, nowadays, that is all I can say. I believe the breather should be quite short, though, probably a bit less than the length of the piston stroke, so the crankcase pressure on the down-stroke can blow surplus oil out, but not suck it back in again on the next  up-stroke!