Sorry to here Your story. This CE thing makes me mad...but not much to do about it. That's why one cant buy a RIB without CE mark from UK unless it older than june 1998....
You are right, technically you need to aftermark it. Is it worth it, that's another story.
These people can do it in UK http://www.gablemarine.com/
but for sure You have similar firms in Germany, might be cheaper that way. Possible the cost can be several K euros, at least that would be the case here.
Think You have a case against the middle man, if You have a sales contract with the details, you might be able to cancel the sale? Or You can sell the hull locally as a work boat.
Or then you forget about the real HIN ( i did not suggest this ...) and keep it as a 01.1998. The fact remains, the expensive CE marking/ certification will not make You boat any safer.