Since that brand is a UK marketed, did not know much about them. Seems like it has a Chinese db152qmi engine. Here is the link I used and viewed the specs pg:
http://scooter.co.uk/125cc-scooter-spyder.html
The noise and running you are experiencing would indicate that somehow you have reduced or eliminated oil flow / pressure to the engine-- but that is strictly a WAG-- if the exhaust is loud enough it might be masking the noise until it shuts down.
Could you remove the cam cover, start the scooter and check to see if oil is flowing to the head?