Prices for 2 timers Tuning? Or even possible at Cagiva Roadster 125

Hello guys, drive a Cagiva Roadster 125 with 12 hp. Got the problem that the existing 6 gear is not used at all. So if you switch to the 6 in the 5th at 110km/h the complete motor falls like in a hole and it runs between 90-100km/h. Have let me tell me from my dealer that my machine is actually an open version (around the 30ps) and therefore the hole. After my question it is possible to do a motor tuning or conversion, that you can also use the 6 gear I’d like to know what it would cost for an air-cooled 2-stroke engine to get a little more power out of it. I don’t want to drive 150 or 180km/h with the ding (even if it was possible) 120-130km/h would already be enough for me to not drive on the highway from everyone else. I’m not going to drive 150 or 180km/h with the ding (even if it was possible) 120-130km/h Truck to be overhauled :-/ Looking forward to reply Greeting