First thing to check would be valve clearances. Take the 4 bolts off the valve cover and check clearances with feeler gauges. Should be .15mm on intake, .20mm exhaust.
My generator would occasionally kick back/lock up when the clearances were wrong.
I have seen a few (especially commercial machines) that end up with a clagged up compression release on the cam gear.
How has the maintenance been? Sometimes its simple as being a bit sludged, other times there is a spring issue or other problem.
I normally do not like oil flush additives, but would suggest drop the oil, fill up with some cheap oil with oil flush and run for 10 and drop again if the valve clearance doesn't help