You only checked the clutch, also known as the fork, but didn't check whether the clutch plate was worn. There is also whether the carburetor is blocked and whether the air filter element is blocked.

Clutch slip or gear slip on the rear wheel bearing. I know that thing on the bike is called gear, but I really don't know what the third wheel is called. It's the thing that transfers power from the sprocket to the transmission wheel. See if there is anything wrong.

Is that car a small chain machine or an ejector? Check the connecting rod bearing under the piston again, and then check the small chain or ejector pin bearing. I am a motorcycle enthusiast and like to play with upgraded cars. These are basically the problems of upgrading the car. That's all I can do for you.