Recently i purchased an 80cc mountain bike engine from ebay and waited about five days for it to be delivered. It took about a good 3 hours for me to install this kit on to my bike and about half an hour to do a few tweaks so if you wanted to do the same i will supply the link for the engine below
http://www.ebay.co.uk/sch/i.html?_fro... plus i will walk you through this step by step tutorial on how to build your very own.