Whizzer with a chain and sprocket

[bbeards2]

I've been thinking more and more about embarking on a new build. The style of bike I would like to build, resembles a 66s cafe racer. Before I start I was wondering if a whizzer engine could have a pulley that goes to a sprocket? Next what size gears would I need to run on the bike to achieve normal speeds considering I would be using a 20 inch wheel instead of a 26.

Thank you