Installing a clutch on a small weed wacker engine?

[Oblivious mechanic]

I am very new to this hobby, and I need a little help getting started. I am trying to us a weed wacker engine to power my bike. I want it to be chain driven. So I would need to some how attach a sprocket and clutch to my engines output shaft. The only problem is that I can not find a clutch small enough for my engine. The output shaft is way to small for the clutches I see online. Any help or advise is greatly appreciated. I think the output shaft is a 5/16ths shaft.