it should just take a standard spark plug with 14mm threads, shouldn't matter whether it has a tapered plug or a plug with a seal washer, just get the ignition you can find for a good price...

I don't recognize the engine right off, I don't think it's one of the 16cc engines so it's most likely a 21.6, 23.X or 25.X, to find out if no one else chimes in, if you have a digital caliper measure the bore through the exhaust port and the stroke through the spark plug hole, to get the size use this formula:

3.14*bore*bore*stroke/4

if you measure in inches you will get cubic inches, if you measure in millimeters divide by 1000 to get cc