OK, so just to finish the topic.
I exchange the oil pump and the coupling. Had the engine washed with a mixture of petrol and engine oil. Change the oil filter (needless to say)
As soon as I had the starter run for 3 or 4 second the oil lamp turned off.
I ran the bike for about 40 km today and I observed no problems and could hear no suspicious noise from the engine.
What a machine, I already love it
In conclusion I will recommend that the three bolts holding the pinion and coupling together are checked or exchanged with forged bolts ( mine where specified as strength 10.9).
(The coupling that I bought on ebay was also modified with helicoil in the aluminium part of the coupling, and there were ballbeiring balls inside the coupling springs).
Best regards to you all
Erik