When that valve cover gasket starts leaking, the oil goes down into the plug wells. "IF" the oil seeps past the crush gaskets on the bottom of the plugs, they will foul and fail to spark. Be sure you have new, properly gapped plugs, and be sure to clean out the plug wells with carb cleaner. The best one is Berryman B-12. Tighten the new plugs correctly & get that valve cover gasket replaced! It's about $45.00 + silicon if you do it yourself. Do the plugs first. If it starts, do the gasket.