does anything like this exist?

A 3d motorsports clutch is ehat your after, but they get spendy.
Go to the 6 1/2 minute mark. He builds what I call an idler arm clutch.

Watch the entire build, but the idler arm clutch is what we used back in the 1950s with a belt drive to the rear wheel.
having built mutliple 2 and 4 stroke bikes, including china doll motors, and custom jackshaft 4 strokes using a honda gx160, im wondering if there is any product, or parts that could be used to acheive this build. I don't want my bike to go more than about 35 mph, any faster and its sketchy, and too low torque for steep hils where I am. My bike built with the big honda was powerful but all but the best custom motorized bike wheels would break under the strain, and the centrifugal clutch was a nightmare. it had plenty of power but it kept breaking parts. What I really need is a 49 to 80cc 4 stroke engine, with a manual clutch and a single gear ratio, much like the 2 stroke engines, the 49cc would be economical and super reliable, and would be powerful enough to keep up with a china doll 2 stroke with the muffler cap removed while also being kind to budget wheels and easy on the rest of the bike. The manual clutch would make the bike much easier to ride, and remove the constant worries from a go kart centrifugal clutch. it would also be compact. again, I think that 35mph is plenty, at least where I live, Ideally it would be more like 28 miles per hour. if you need to go faster or futher, a motorized bike in my opinion requires too much money and work to be as dependable as for example, a 125cc moped would be. Im just wondering how to put this sort of dream bike together. Any ideas? another great thing about the four stroke's are that they deal with hot weather and slower riding better since they don't require fast speeds to cool the motor.
Have you looked at kits?