I would like to make the K3 usable for long trips, 2000km each way perhaps.
With cruising speeds at around 130km/h on the german highway.
Which modifications would you advise me to make?
Seats, handlebars, windscreen or what?
It is usable right now for long trips. Bring chain lube to apply at the end of each day's ride. The machine is tough. How tough are you?
If you still have a seat with half the foam removed, you will be, er, tender. The 400's narrower seat is NOT for touring, unless you have a very narrow pelvic bone.
Youth has resilience. You can take punishment and recover quickly to the derriere abuse, constant wind pressure on your chest, constant helmet lift, and pressure on your wrists and arms.
A good seat and/or some lambs wool will help your bum.
A good windshield will take wind pressure off your chest and keep the wind from trying to take off your helmet. Some designs are better at reducing buffet and buffet noise, which can also be fatiguing.
Handlebar position that allows you to sit upright without using your wrists and arms to hold up your upper body will reduce hand numbness, lower back, arm, and wrist fatigue.
The difference may be that you are ready to explore, investigate, and party at your destination. Or, that you just want to lay down and sleep when you get off the bike.
If you make these changes to your bike, all the cafe bigots on this list and elsewhere will ridicule you mercilessly.
Comfort has it's price.
Cheers,