Those only fit cars that have drum brakes on the front, so make sure you install the retrofit kit to swap from discs to drums.

You may want to run it on a hose and see if you can get more timing in it at idle might get it where it passes noise meter. If so get a ignition box you can program with a map sensor. My boat idles at 45* timing with the standard cylinder heads it calms it down a bunch on the noise.

idel,s at 45 deg,why?

It's already been mentioned, but slip in mufflers are easy...

Or if you only need a couple DB -- why not some external flappers, that should help a little?

How are they measuring you? 1 meter back, and 1 meter up from the transom?

If you don't already have them, try some of rubber flappers. At idle it will direct the sound downward toward the water and may help you pass. Might be worth a shot.

You can close the throttle blades and it will lower cylinder pressure with more advance at the same engine speed. Kind of like a vacuum advance un plug it and the engine will slow down.
Did it on my engines it calms them down and makes the exhaust non ear piercing.

I went in where my overboard water bungs were, took out the fitting and drilled a 3/8" hole into the inner exhaust pipe (was about a foot from the exit). injection of the water dispersed the sound and made it a lot quieter without restricting the exhaust any more than necessary.