the ezm qmatic transmission will bolt on to the 79cc and the 99cc. you may have to grind your shaft down a teeny bit depending on the shaft size I think. for the 99cc, there is also a transmission available at www.affordablegokarts.com
that is designed specifically for the 99cc. I suppose it would also fit the 79cc. the agk tranny is much cheaper than the qmatic, but you have to buy a clutch separate. there are no complete kits available for the 99cc, so you will have to hook up your own throttle linkage and exhaust and also find a motor mount. the 4 stroke kit mounts will work but may require some modification. you will also need to remove or disconnect the governor. most here just disconnect it. ezm, the company that sells the qmatic transmission, sells flex pipe exhaust for like 36 bucks, or you can make your own. but you certainly do not have to build a transmission completely from scratch. check the diy section for more ideas and info.