Replace what you have with similar is usually logical. Modern laundry usually has a vent, so at least a gap at the bottom of the door is necessary. Furnace room will have more important code requirements. Old furaces and water heaters drew air from the room. That’s no longer allowed in new appliances. So if they do draw from the room, you must have ventilation. If not, it doesn’t matter / isn’t required. Either way, that doesn’t prevent you from installing solid doors. Also, you can vent through the interior wall if you don’t want to cut into your door.