You're not the first
Personally, I'd clean it and run it again. I've run worse when I was a kid…
yes and this is not first of my seized piston.
The main problem with this cylinder was that it always had seizes after reboring to last piston size.
I think that guy who rebored it was made some mistake.
When I got it after reboring I carefully run it in for 1000 km. There was no air leaks, jetting was right, plug colour nice, ignition timing ok.
And when I start to ride harder it starts to seized.
In total it was ten seizes and this one is eleven after what engine not starts again.
As I mentioned above this is not big problem as I have Malossi 112 in good condition. So just threw it in. Also this PK is not my main Vespa, more likely workhorse for riding to work or to give it to friend for a short ride.