[50nuts]

Classic Honda sells the Kitaco 75cc big bore kit. It is about a third of the way down the page.



http://www.hondaminitrail.com/XR50.html



*edit*



minibikeoutlet does have better prices.



Here is the Sano Kit...

http://www.minibikeoutlet.com/detail.php?catid=3&prodid=218



and the Kitaco kits...

http://www.minibikeoutlet.com/detail.php?catid=3&prodid=608

http://www.minibikeoutlet.com/detail.php?catid=3&prodid=773