12kg is what I was told by Mr Godet (RIP).
FWIW I would take care and keep a close eye on the double sided belt. Set up is critical. If the alternator / Dynamo is mounted too close, the belt can crush and the bearings in the alternator can fail due to the stresses applied as things expand / contract (which they do on such a huge lump). If it’s mounted with too much clearence, the pulley can skip over the teeth, also destroying the belt. This is not a consideration with the stock set up as the sprocket can move in / out of the chain to cope with expansion and contraction etc. I had that double sided belt set up some years ago and destroyed two belts learning the above points !