Coles Power Models used to peddle a casting set called the "Vesuvius" that was designed to power a bicycle (It's currently showing as not available). I think they also sold the plans separately at one time. The plans, at least, might be worth seeking out - rather than trying to re-invent the wheel.
http://www.colespowermodels.com/index.html
http://www.colespowermodels.com/index.html