melted piston

Hi need some help here please i have a 50  special that was running on on racing crank and a polini 115cc kit 19/19 carball great then the piston seemmed to melt and it stopped of course. wanted to make sure every part was still ok so have put the 50cc piston and barrell kit back on and it tries to fire but will not run. is it not liking the racing crank or do i have another fault also any help would be great