Hey Jonas,

as a first step you can adjust the CO value. There is a special mode in the on-board computer where you can make the fuel ratio fatter, i.e. increase the CO value. This reduces the surging a bit (it is caused by the very lean fuel ratio)

Powertrain clearance increases the problem. You should also check the chain tension and the rubber absorbers (Ruckd�mpfer) in the rear hub. They can be modified easily with 4 pieces of an old inner tube.

These 2 mods will not affect the legality of your bike. It's just an adjustment of the engine and powertrain.
You will find some instructions here at XT660.com or at the German forum (XT-660.de)


Regarding your original question: The liability insurance has to pay in any case. Might get you regressed later. To do this, however, the KevMod has to be found first. It's not as obvious as an illegal exhaust or changes to the suspension.
Most riders have some -more or less- illegal mods at there bikes, but who cares?

And by the way: welcome to the forum

Cheers
Werner