I seem to remember back in the late 70s, early 80s when noise limits and testing was first introduced by the ACU at motocross competitions, many competitors knocked a hole in the bottom of a coke can and gaffer taped it to the tailpipe...
It got them through tech inspection. 
82db is very quiet for a two stroke though.  Is that just exhaust noise, or total noise like piston slap?

Thanks for the replies!

I've forgotten to mention that the exhaust must look original.

The bike is an early model and has this muffler:


The origianl muffler was broken as the pipe part was fully missing and the exiting hole had seen some hits. I've recreated the pipe with perforated shit metal and stuffed it with packing material. This gave the result of 93dB.
At the homologation I've got also a defect that the end of the pipe is not "original" or modified. So I had to change it wit the full oiriginal one above....now the noise is even higher than 93dB.

I would say the noise is measured around the exhaust exit and not all around the bike...if it is the latter it might be an issue as the engine is pretty noisy.

I will go to a workshop with a lathe today and I'll manufacture multiple reducers to put inside all pieces of the exhaust, I don't have a better idea at the moment.
I will anyway bring the bike to the test on a trailer, so I won't ride it for a mile with the clogged exhaust.

A lot of noise comes from the air filter/air intake. 

I'd try laying a sheet of bulk air filter foam under the seat and over the air inlet to dampen sound. 

www.amazon.com/SHUNTING-Filter-Sponges-S.../dp/B0B7734HK1/?th=1

That outlet is the same that is fitted to the DT100...  Recognise the part number.

And yes, the DT100 is a very loud exhaust.  That outlet does nothing to reduce it.

Back in the day 70’s we would stuff steel wool in our Cherry Bomb Glass packs to pass inspection 

Great idea HT1kid... steel wool in the stock insert might help a lot.

Another challenge is I believe that in 1975 the US standard was 86 dBa. 

"Another challenge is I believe that in 1975 the US standard was 86 dBa." 

European bikes may well have used the same exhaust system. However, I suspect that what we have here is current regs being retrospectively applied (due to first registration now) to a vintage vehicle. Any vehicle registered back then, when new, is probably not subject to current regs. Saying that, the Swiss do have their own little quirks! 

Indeed.  I wonder what the db level of the AK47s and other full automatic weapons that the Swiss population seem to enjoy enormously top out at?

Thanks everyone for the tips!
I've stuffed some foam in the intake snorkel, I've reduced the diameter of the silencer inlet to 1/3 the original cross section, I've reduced the muffler cross section to almost nothing...finger crossed for the next inspection!