The engine you have is a Type 133 engine made for the military, many of these keep turning up for sale, usually still in the original box never used, unfortunately I haven't found any reliable source of information for what the military used them for, that include the sites that mention Vietnam and the wrong info given on some sites mentioning WW2 (these engines didn't exist then).
The Type 133 engine has a tapered power output shaft, which is the wrong type for the bike kit gearbox which uses a clutch that fits the "D" shaped power output shaft.
You are most likely to find these kits on eBay if you keep looking on there, in fact there are three available at the moment, some of which have been on eBay for a long time.
Orline kit, missing tank, throttle cable & proper exhaust (it may be hard to find these parts), has best offer available;
http://www.ebay.com/itm/112536567748
Orline kit, missing tank and does not spark, a bit expensive but has best offer available;
http://www.ebay.com/itm/222581900155
Chicken Power branded kit with a bike you may not want, you could ask if they will sell the kit without the bike, also has best offer;
http://www.ebay.com/itm/322656615139
David