I cant remember the last time I saw a plastic plug in the bottom of a pickup...maybe the last year was 1974 I cant remember but if you were running fine then it dropped off I wouldnt guess the pickup. I would guess a cam bearing walked out on you, filter stuck, pressure relief stuck, blown out gasket on the timing cover. I would measure from the bottom of the block to the end of the pickup and then from the top lip of the pan to the bottom of the pan. This will tell you if your pickup is not screwed in all the way.