the cheapest (as long as the seller hasn't decided it's a collectors item or something), might be to find a fork off a huffy/jesse james/wcc chopper (not the Schwinn occ chopper). most likely have to buy the whole bike. don't know where you are, but I've seen them on craigslist here down to around $40.
being as you're saying motorized though, you'd probably want good brakes and the wcc fork has nothing for that. it's a fairly sturdy fork though, and if you're adding an engine you may have the capability of adding a disc brake mount.
I'm talking cheap enough to experiment with though. for more expensive (and probably better thought out) options I'd probably check one of the motorized bike forums.