I built a mix-n-match bike out of the wreckage of my old bike and a couple of scraps I was restoring once...if necessity is the mother of invention, desperation is surely the father.
Although the accident that near killed me smashed off the muffler and stripped out one of the cylinder bolts, the engine itself from the '89 Golden Bullet worked well enough (an A3, I believe it was) and so I took it off, and built a 'ped around it from the wreckage and the salvage bikes.
Let's see....engine/trans, the A3 from the '89 Bullet...the DelLorto carb was cracked, well, the plastic piece on the bottom was, so the new carb was an Encarwi (I think that's how it was spelled) off of gods-know-what...I've no idea.
The rear part of the frame was a '74 Tomos-Koper, a bit rusty and ugly, but solid, but the front end was trashed with bent forks, so I took the front end off of a '78 Puch Maxi and used that, after doing some mix-n-match to find something to replace the bearings so it would actually turn.
Kept the Puch headlight, but had no tail lights at all, so I pulled one off an old Yamaha, and built a mount for it on the top of the rear fender, only to find that the Tomos-Koper frame had no wiring harness, nor any wiring at all still attached to it.
Being that 6V systems suck, I stripped the coil and regulator off the Bullet and rewired the whole damn thing, using about half a roll of electrical tape.
The Puch front end only had the left handlegrip and brake, so the right grip, brake lever, throttle, etc - wound up coming off the Bullet, as well.
It was pretty mix-n-match, a frankenped, if you would, but it did run, though poorly, and tended to throw the chain or stall out whenever it pleased, and without any muffler, sounded pretty nasty as well...but it worked, long enough to buy another 'ped, at least....so like the man says, you never know.
One thing I did work on, and still have the design for, over that - is an outrigger wheel for a 'ped, this due to the fact that if I hadn't gotten a trike, once the leg had to come off, it would have been somewhat difficult to ride a stock 'ped in that shape, and I figured that it might been needed.
Basically it's a metal A-frame type of gadget that bolts to the rear wheel and the main bolt that holds those two bars on (which means you couldn't use it on a mono-shock 'ped) and has a small wheel from one of the better electric carts on the other end.
Although it's a bit off-topic, you can get a 'ped that'll work for you if you cannot use both legs, the Honda Gyro has a variant perfect for it called the "Gyro Up" for about $3,500 US (374,000 Yen) - plus shipping from Japan.
( http://www.hondagryo.com )
Check it out, it's pretty neat.