Your bike is now 30 years old. Whatever the original specs were back in 1979 have no real relevance now. What you should ask is what were the noise limits in Greece in 1979! Only the authorities there will be able to help.
In the UK in 1982 new bikes had to conform with 86 Db (a new fangled EEC law) before that date I suspect there was no specified limit.
In 1979 I had a Ducati 900SS with the original Conti 'silencers' which my parents could hear me start when I was up to two miles away. They were standard and accepted at the time though you could opt for Silentium silencers.
If your local vehicle licencing people need to know the noise level then they must have a noise testing facility somewhere. I suggest you get your bike checked by them. If it has standard or close to standard exhausts then it should be able to pass whatever law Greece had in 1979 it might even pass the current limit (80 Db I think). Of course it cannot be expected to pass limits set after its manufacture though you might be lucky.
Whatever the specs were in 1979 means nothing as you could ride it with no silencers whatsoever if you so wished, if you see what I mean?
