No performance in warm condition

Hello guys , I have a problem with my Peugeot speedfight 2 50 ccm ac “again”. The 2 wheel mechanics in my area have only in October again time so I have to solve it myself somehow . My scooter runs no matter how long it is after the first attempt with the kickstarter and runs very calmly . Then he drives his 50/60 kmh on the straight or also up the mountain .Everything great until it gets warm . As soon as it gets warm he drives on the straight still 40 kmh and on the mountain he remains one fac h stand even if I drive up with momentum. I guess either the oil pump (mechanical) or it can be the gearbox? Or yet the drive. I am at the end with my knowledge about 2 wheels maybe someone has had the same problem here and also found the solution. I am grateful about every tip. Many greetings and a nice Sunday evening.