Change the oil to a good grade of 10w30. It does not have to be synthetic, they run fine on regular 10w30 oil.
I do not recommend putting kerosene, diesel or anything like that in there for a 'flush'. You get the old crappy oil out and it will probably be quiet after it warms up.
If you want to add 2 oz. of Sea Foam to the oil that would probably be o.k. but keep a check on it, the Sea Foam will eventually evaporate and you will need to add oil. Almost all of those engines use oil while they run anyway.
If it were valves or seats you would have a noticeable lack of power. And there is no valve adjustment to that system. You torque the rocker arms and the lifter takes care of the clearance.