You're our resident expert on Danish law. However, I've spoken with a number of riders in other EU countries, and the UK. The common thread is that vintage/antique vehicles are usually given a fairly lax standard when it comes to compliance and a 40 year old bike is more than old enough to qualify. I wouldn't expect to find many experts on these bikes working for a unit of government anyplace outside of Japan. The bikes were relatively rare in your part of the world. However, playing devil's advocate, assuming that you have to deal with an "expert" (whatever that actually means at inspection time), that individual should be able to determine that both the VIN tag & matching number stamped into the frame are original from Honda. Having the bike inspected in its raw, unrestored, state might also help you present your case effectively. It could be reasonably argued that no one would bother to take all the trouble of creating a fake VIN on what appears to be a rusty 4-decade old piece of junk, that's worth less than 80EUR and very likely cost more than that to have shipped.
The VIN tag digits will be stamped from the reverse side and thus raised, unlike anything a shadetree mechanic could do with a die set or even a trophy shop for that matter. VIN tags found on US-spec bikes of this vintage clearly state "THIS VEHICLE CONFORMS TO ALL APPLICABLE U.S. FEDERAL MOTOR VEHICLE STANDARDS IN EFFECT ON THE DATE OF MANUFACTURE SHOWN ABOVE:" Combine that with the original drive screws (the rivets holding the tag in place) showing years of rust patina and clearly unmolested since day one and your argument becomes quite convincing, especially since it's the truth. If you are legally allowed to import US-spec bikes of this vintage in the first place, that should give you some negotiating room, if you need it. I mean, what is the likelihood that anyone you'll have to deal with could be an expert on US market models that are probably older than he, or she, is?
But then, I do realize that this is all conjecture on my part, I could be dead wrong.