Yesterday evening I checked the coolant container.
When I opened the cap on the container, it released much pressure, making a hissing sound and coolant level increased in it (I didn't even tried to start the engine yet).
I did not see any bubbles in the coolant while tried to start the engine. The funny thing is, the engine started quickly... I left the coolant container open to see inside it while the engine reached the normal operating temperature, but there was not any bubbles. The coolant is clear, it does not contain any oil.

I checked the engine oil as well, it does not contain any coolant, but lighty smells like gasoline. Could that be a root problem of hard cold starting?

Another question: is that possible, that if the scooter is tilted to a direction anything else like normal level, then it can start easily, and tilting back to normal causes no start?

If everything goes well, I receive the 150 ccm cylinder set today and the gasket set tomorrow so I can disassemble the engine on the weekend.
I will keep you posted.

I keep my fingers crossed that everything works very well ...
Unfortunately, this is all that i can do.

Thanks, it looks like you helped.

I swapped in the 150 ccm cylinder and piston with the new gaskets and the engine started easily.
The old cylinder head gasket looked like this:


You can see some burnt black signs on the gasket at the exhaust valves' side...

After I finished the engine, I broke a fuel line coupling on the top of the fuel tank, so I still couldn't put the sccoter together.
Now I'm waiting for the tank part. I hope I will receive it today so tomorrow I can come to work with the Leo! I will keep you posted...

Thank you very much for your kind help so far!!!