Generate electric from treadle sewing machine?

I have two working antique treadle sewing machines. Without altering or damaging these machines, I'd like to harness the belt driven power of these treadles to create electricity -- something like the Pedal-a-watt unit for bikes. How can I build such a unit? Pedal a watt says it is made of off the shelf components -- surely I can use these same components to make myself a power generating system.