I have been messing around with DIY Weed Whacker friction drive build and the process has been fun. Since I my budget for builds is usually $0 I usually repurpose and reuse things as I tinker in my secret laboratory. So today I had a great for a low cost bolt on kit for motorizing bikes. Make a kit that utilizes the gearbox/gear head of a weedwhacker. The gearbox on a weedwacker could be mounted in a variety of ways, to accomadate friction or chain drive and the selling point of this kit is that the engines would be dirt cheap, the gear box are pretty universal how they mate to a string trimmer engine, so you could use almost any 10 dollar craigslist trimmer for a motor, and because it will be using the flex shaft you might be able to come up with an inframe mount if you wanted. Also because the bolt on portion is the gear head, you would not have to strip down the engine and figure out shaft adapters and stuff, and many already have a centrifgal clutch that would function just fine. So any way I have a string trimmer gear box, and I have changed from trying to come up with a decent engine mount and drive set up to figuring out a decent gear box mount and drive setup, then the engine becomes plug and play, easily swapped out and changed with out having to reinvent the wheel.