Engin Seizure

Hi all I have recently rebuilt my 1979 Vespa p125x I have a New DR180 kit and new crank.
I have done about 100 miles on it with no promlems then experienced my first engine seizure (very scary).
It restarted ok and I drove it home for about 2 miles the engine now rattles a bit.
Can anyone give me some advice? Please.