Peugeot Speedfight 4 50cc LC 2017

Hi Guys, recently bought a 50cc for my son and am no stranger to 2strokes given I have a few MX bikes

this one has done 900km and was bought non running but had just passed the MOT. Got it home and spent an hour to get it running after cleaning the carb and fresh fuel. Took it out on the road and literally 50 yards engine ceased.

put some 2t in the piston and left over night to cool which freed it up. Replaced the piston and cylinder and thought all would be good as the piston was damaged.

however, now once all back together the engine ceases after a few turns. Piston runs free when I release the cylinder from the crank casing so I’m thing it must be the crank

Any ideas would be greatly appreciated?