The engine rebuild should have taken care of any oil prime issues provided the right parts were used or not damaged during assembly. If prime at the filter doesn't work, the preferred method is to remove the oil pressure relief valve on the front timing case (below the water pump/above the dist) and shoot some heavy oil into it like 90-140 weight. Be careful not to lose the relief valve spring when removing. If you continue to lose prime, at your next oil change look into the drain pan for the pickup tube and give it a wiggle test. If it moves then you probably have a crack or busted solder joint at the oil pump mount flange which can cause loss of prime. The only "common" side to this problem with the N series is it is a major pain to fix, not that it is a feature of the tractor.