Problems with my Gilera Runner SP 50cc (1997) at 30,000 km?

Hello everyone, I have a 1997 Gilera Runner SP 50cc with a C142M engine, which I bought two years ago. The previous owner said he replaced the gearbox before selling it. Initially, the scooter ran great, but last year, the problems started: Starting problems : The scooter wouldn't start. After changing the spark plug, it…