How can I put a second cog on the back wheel of a bicycle using an old cog?

Question

I want to put a chainsaw motor to my bike but a still want to pedal my mountain bike and so, some people have some kits to just add a sprocket but i want it to be smaller so i want to use an old bike cog please help remeber the back wheel has gears on the right side this is becasue it a mountain bike

Answer 1

All I can think of is to find another mechanic that has already done this and learn for that modification.  That's an interesting idea.