Hi ,
Well, my first diesel bike (daihatsu) was build with a Guzzi500 gearbox.
The beauty of this "powertrain" is that a single pair of cogwheels sets all ratio's ...
There is even no need to dismantle the actual gearbox for that ! The box I used even had a kickstarter

These bikes are very cheap: I've purchased 2 complete Guzzi500's for Hfl 700,- (about 300 Euro)
The drawback of the Guzzi box is also obvious: the swingarm is -at least for machines > 250kg- too weak and "she" never made it to our RDW license_plate (?) registration.
(I 'm thinking of buying some chinese "punsung" lightweight diesel...for this
The BMW drive-train is much more stable (although it's certainly not what it used to be !)
The 5th gear leaves "some room" for a 1:1 set of cogwheels.
The removal of the fixed (heat shrunk) cogwheel was a true pain in the a...
The production of the cogwheels was not really a problem and I've heat_treated the cogwheels (Arne steel) myself...
and I'm no engineer (that answers the question of selling cogwheels
To be short: the machine does not run Yet...I hope to find some time later this year to "finish" the job...
And if the gearbox survives the first real tests...and I may live to tell... I'll hand over the "machining data".
sjonnie