Marty, I'm sorry. But the only power source capable of generating 1.21 gigawatts of electricity is a bolt of lightning. It says here... that a bolt of lightning is going to strike the clock tower at precisely 10:04pm, next Saturday night! If... If we could somehow... *harness* this lightning... *channel* it... into the flux capacitor... it just might work.
This may help you figure it out, I didn't find any drawings though...
http://en.wikipedia.org/wiki/Flux_capacitor#.22Jiggawatt.22