How to adapt a 4-cycle string trim motor to drive a sprocket&chain?

I want to engineer some way to use a std string trimmer motor (4-cycle) to drive a bicycle chain and sprocket. The geared-down driven sprocket could either then drive a large prop for "air-power", or drive the rear wheel, like the 80 CC PK conversion kits do. I don't have a machine shop or lathe, just basic power and hand tools. for the driver sprocket, I would like to use a std. sprocket from a rear derailleur.
I can't so far think of a good/easy way to connect a derailleur sprocket to a string trimmer output shaft. The output shaft pretty much does not exist...that is the problem. There is just a square hole
into which the square end of the flex shaft fits. Probably it needs some sort of adapter that would
bolt onto the output end of the engine, containing a bearing, in which is a square-shaped "key-
stock" shaft (to engage the engine) which changes to round, and on the other side of the bearing has a short round shaft of the proper size to fit the ID of the derailleur sprocket. This involves some complicated machining. If someone could put such an adapter into production, one could probably sell quite a few. (maybe such a sprocket adapter has already been developed?) The nice thing about this is that my 4-cycle weed wacker has an integral centrifugal clutch, so there is no need for any manual clutch on handlebars. You release the throttle...clutch disengages. Probably all the string trimmers are like this.
String trimmers are designed to produce a high RPM at cutter head, but very little torque, so a gear reduction is needed.



Any ideas/thoughts?

Yea, I had the same thought, just take the bolt that is used for the sprocket wheel and square off the end. And now I am thinking that if I take the engine completely out of the plastic case, the bolt/shaft wouldn't have to be three inches or so long, probably just an inch long. Still, I'd have to make some sort of a square plate with bearing, to keep the end of bolt/shaft seated into the engine shaft. But that is within my means, and I could support it with four lengths of threaded rod on each corner of the plate. It doesn't look like there is enough engine shaft length to drill and tap a hole for a screw whcich would secure the sprocket shaft. How did you secure yours onto the engine shaft?

In the Sears catalog, I see they have a 4-cycle lawn edger. This would be much easier to put a chain sprocket on, I would think. But I don;t

In the Sears catalog, I see they have a 4-cycle lawn edger. This would be much easier to put a chain sprocket on, I would think. But I don't know if it also has a cetrifugal clutch or not like the 4-cycle weed-eaters do. Anyone know? The price was very resonable/affordable, too.