Sounds like this motor will need a few things checked for serviceability , make sure you have a good spark plug first as I've had a few plugs that break down when hot and cause a misfire.
Yes I've seen a few motors that the valve seat has come loose ,when the seat drops completely out you end up with no compression.
It's a possibility that the seat is starting to move when hot, you would need to remove the valves to check.
If you fast forward to 7 min and 54 seconds in ,he discuses centre punching the alloy to hold the seat in.
If you fast forward to 10 min and 48 seconds the video is showing the loose seat.
