be prepared to get addicted and spend alot. those guys doing those speeds have gx390 honda engines(13hp stock) bored out as well as copius amounts of other work. Getting to the 50 mph or so mark aint hard(and that already is AWESOME) but pushing past that can take a ton of work and money. 10hp or so with the proper gearing and tires can get u to 50 mph or so. A brand new 6.5hp honda clone is only 100 bucks with plenty of performance parts available for them.
http://www.oldminibikes.com/forum/classified-section/
almost always has some for sale, as well as an ebay spot that all ebay sales relevant to minis are posted in.