Rebuild worries 😟

Do you have any knowledge of any engine work done by previous owner ?

My understanding is about 350 to 400 hours is max on a 4 stroke trail bike piston ridden easy.

I saw 2 pistons and it refers to engine serial number.
I would talk to previous owner if you can to see if it has been done.
If not take your time doing it .
Clean engine before disassembly.
Do it in a clean organized way.
Research sherco manuals.
Inspect cylinder. It might need to be replated if so send new piston so they can match bore to piston.
Definitely replace all cly. head valves. And the retainers and springs and seals.

Reason is if it breaks a valve due to fatigue it destroys the engine.