Ok I have a 2002 Grizzly 660, bored it over to a 686 because it threw a hole in the piston. Has whole new jug, sleeve and piston. Timing chain is new too. But it idles rough when it’s hot (few minutes of running) and then has trouble if not even wanting to start. Everything is new except the very top end ( valves, cam). Would the valves need adjusted and or replaced to solve this?

(Also off subject, would I need an aftermarket radiator to stop the over heating issue or will new valves solve this also)