Beware of plugs not specifically designed for the engine that may fit the same size hole but not work optimally. Look up the stock NKG plug on a spark plug conversion chart, easy to find on the Internet, to see what else is similar and compatible.
The air filter on the MK1 and MK2 bikes is a foam type that is designed to be cleaned and oiled. Pick up a K&N air filter oil and clean with brake cleaning solvent and then later warm water and detergent. Let fully dry and then re oil with tacky to the touch, K&N air filter oil.