Are you sure the dipstick was replaced with a plug? In most engines of this size, there is no dipstick. There is a plastic oil plug sticking up at a 45 degree angle from the crankcase. You unscrew it to check the oil - you can put your 5th finger in there to see if the oil is up to the lower rim of the hole - and you also add oil through the same opening. Draining the oil is done through a different, usually metal plug, lower down on the crankcase. Hope this helps you!