Cable, pressure plate, inner hub, actuator, busings, and shift components are all wear items.
They should be checked every year, some replaced every year
Old cable, welded ball/rod, bad actuator, warped pressure plate, warped hub, grooved basket, sloppy basket bushings, worn fiber, warped steels, discolored steels, burnt oil, burnt wood smell, and take a lot of pics.
I believe millennium has pistons on hand. They will measure the pistons and match them to the cylinder
Also if the motor seized I'd be pulling the crank out and inspecting that. A hot crank is never a good thing.
If the wife and son enjoy it keep it.
But yeah I'd suggest an inspection of the entire motor.
I'm sure there's more then a few worn out parts.
How into it is your son? At 16 you couldn't keep me away from my toys. I'd wrench at it every chance I got.
He may be willing to do a fair bit of work.
Send the engine out, you guys can toy around with the chassis. I'd say cables, bearings, and brakes are an easy job to tackle between the 2 of you.
And there's always this place to refer back to for any help you might need.
Dude if your in such a time crunch you could've ordered a honda for 20 bucks or got the 450 and tried it. But your waiting for some one to answer your question.
You selling the bike, your going to get paid at the end.