I was told from my dealer that riding the GV 250 at 65mph for more than 15 - 20 minutes is very stressful and not good for that motor.
For some strange reason, you will hear a lot of bs if you talk to delers and, surprisingly, sometimes to service personnel.
Avoid tech talk with dealers completely and judge service personnel by yourself.
I have 250 cc scoot that happily goes around highway at 90 mph for hours without any problems.
I had in the past 200 cc scoot that did the same.
All scoots run perfectly and have regular oil changes.
Every vehicle I have ever owned was driven at top speed for long period of time with no immediate or cumulative ill effects, including sports bikes and small 2T/4T scoots and bikes, water, oil and air cooled. Not even overheating. Never a mechanical engine breakdown.
Your piston problem was unrelated freak incident at that mileage.
It is as simple as that.