Second jimmy first issue

[2xJimmy]

P0300 code showing, idling rough at low rpms, no water in oil, no excess fuel smell, overheated the other day first was able to shut it down and let sit for cooling period on the way home temp flew past normal operation and it shut down. At both times it sounded as though lifters were tapping at a very high rate and loose beyond repair now it has no noise and fires right up but still gets warm fast... possible issues? I owned a 93 jimmy and jumped on the opportunity to snatch this 2001 up, because they are resilient little trucks but if I can’t fix it then I’m afraid I’ll have to get rid of it... I honestly don’t want to lose the little truck had some plans for it so any help would be much appreciated