Dud scooter, or dud dealer?
Technically/legally, you haven't got much of a leg to stand on unless you've got something on your contract that says that the vehicle will do these speeds - and given that they are only supposed to do 50kmh legally, you are going to find that difficult. The only guarantee is that it should go 50kmh, and if it can do that, then its fit for purpose. What has been done to get your scooter to go over 50kmh?
There does tend to be a lot of variation in performance in scooters, particularly at this end of the market. But I'm doubly intrigued about nuts coming off in the "gearbox".
If you are looking for something to go reliably at 75kmh, I'd strongly recommend thinking about getting your license, and a bigger scooter. A 125cc would easily satisfy your needs.