California (and most states I think) allow you to drive mopeds/scooters/vespas/gas powered bicycles without a liscence if the engine displacement is under 50cc 's.
There's a lot of 2-stroke kits you can buy off of ebay (really cheap chinese motors) like this:
http://cgi.ebay.com/BICYCLE-ENGINE-K...QQcmdZViewItem
As of 2005 they are no longer legal to import due to emissions laws and I imagine being so cheap they won't last very long (or work at all) so buyer beware.