Moped over-revved?

Hello everyone

I own a Kreidler Florett K54 built in 1964 when I give it full throttle I reach about 50kmh but it sounds as if another gear is missing or it is over-revving, meaning the 4th gear would be missing but I only have 3

What can I do to stop the engine from over-revving so much?

I've also thought about putting a larger pinion on the front and a smaller one on the back, would that help?

It is also the case that if I stop and do not give any gas for about 10-15 seconds, the moped simply goes off and then does not respond anymore no matter how often I kick it

(all original)

thanks to everyone who can help