You buy the clutch you need as a complete assembly. You also need a special clutch removal tool. Many people make this tool themselves but here it is. Just search YouTube for clutch replacement videos for the Honda horizontal engine. Also called a pit bike engine. A monkey bike engine. Or by cc 50cc, 70cc, 110cc. You do not need to remove the engine from the bike as long as you have access to the clutch cover case.
http://www.ebay.com/itm/CLUTCH-HUB-R...64fcb3&vxp=mtr
http://www.ebay.com/itm/Manual-Clutc...item43dd7aa3ec