No guarantee case?

Hello dear forum community, my name is Martin, I’m 19 years old and I have a problem. I bought a 50s Supermoto and had a failure after about 80km already. After a cozy 30Km tour, I turned off the machine in the yard, wanted to drink something, when I wanted to drive again, nothing went. The kickstarter didn’t want to use anymore and the e-starter I didn’t want to use. So I tried to contact the dealer because I wanted to know when he had the machine there After 4 months, a registered letter and about 30 phone calls and e-mails I was able to contact him. The next day I brought the machine at my own cost the 130Km on the trailer. He suspects a transmission damage and has to send in the engine, was all he says about it. After 4 weeks I get the info (after I had called more often) that he doesn’t know anything. 1 week ago, about 3 months after I call the Maschi. I lost the engine and went there. Two tried to reach it before failed. Then I got there to hear that the gearbox was defective, and the repair was not worthwhile. A new engine would be cheaper, because the company, where he sends his engines to give about 60€ per hour. However, the gearbox is not a guarantee case, because the teeth of the gearbox were bent by switched off. The repair will give me about 300€ to 350€ He has received the report by e-mail and will forward it to me, he has said. Now my question to you: What should I do? I can’t understand how this should have happened, since only I drove the machine and drove the machine normally to the last meter. Have you ever heard that teeth bend? What right do I have now?