not that I know of but they are easy to make...Most of us here have made a dozen or so.
its just a couple of angle pieces and a few straight bars and an L bracket or two. Then run your throttle cable back and marry it to the one from from the whacker. put a spring to hold it down and you have an engine. You can add a gravity clutch if you want. I would go for that option if I were you.
If you get a ryobi with a clutch, after you remove the clutch parts a bmx axle peg will screw right on... If you get one without a clutch you can make a drive wheel easily enough. I have both kinds by the way.
The frame is to keep the engine from flexing then you just attach it to the bike with a hinge. Then use a spring to keep tension on the wheel. If you decide to go with the friction clutch you need scissor henges to keep it from shifting around and a lever or cable system to lift the engine. Nothing hard there either.
If you need any help just ask we are all willing to give you advice. You can do it in a Saturday easily enough.
All that said the easiest one is the front mount on a mountain bike. It is super easy. It is also the most powerful I think. It's like a front wheel drive car it just seems to work better power to weight for some reason. Thats just my opinion.
And if you go front wheel drive you can keep your rear wheel gears. It is a much simpler build all around.
I am curious, since i might just build kits. How much would you be willing to do. if i supplied you all the parts and detailed instructions, Would you be willing to assemble it and do some cutting, filing, and drilling to make it fit. All engines and bikes are not the same.
also, how would i make it work with a weed whacker with a cable as a drive shaft?
and to answer your question, i'd be willing to drill the mounting holes and do some cutting and filing if i bought a kit to mount one