no, your question was,
"How can I figure out the exact VIN-Number..." and my answer was correct. the 7 digit number IS the VIN assigned to the bike when it was built.
what is the point of attempting to convert an old vin to a new one? what are you trying to find out? just to run it through some online vin decoder? 100% probability that even if you were to find some way to "convert" the number to some fictitious new number, the decoder databases won't have any info about the vin....because it never existed in the modern std format.
and, in order to covert it, you would need all the info that a vin decoder might give you. look up VIN format. it is just info about where made, what county made for, etc. you know all that already.
realoem.com recognizes the 7 digit number and provides month/year manufacture and what model it was. what are you looking for? original "options"? just contact BMW with the 7 digit VIN and they will give you the info they have. folks in the old BMW car community do that all the time.
i must be missing the point of this (pointless?) excercise....:-)