edit: there are rules regarding motorised pushbikes in Vic that you need to familiarise yourself with before pouncing on an ebay purchase. In summary, the 70cc two stroke engines that are everywhere online are TOO POWERFUL to match our local requirements. Also: two stroke engines are rubbish. Do the homework yourself first.
My old bike. It's the one I got very cheaply when paying for the 2006 Great Victorian Bike Ride. It's actually a pretty good bike. (cheap) Shimano derailers, a reasonably light weight steel frame. Nice hybrid setup. No rubbish bike. So anyway...it's getting a motor. The bike ready for its operation, mounted in the car-bike-rack which is clamped up against the work bench.
My "new" bike. A mate went overseas so I got it from him on the cheap before he left. It's actually a pretty sweet machine. 
A 50cc 4 stroke pushbike motor kit. 
Seems pretty nicely packed. 
Quite nicely packed in fact. The engine mount bracket is seen in the bottom right corner of this image. The entire kit was designed around standard 26inch frames. The bracket is fully adjustable, however I have a 27 inch frame, and I had to modify this bracket somewhat later.edit: I didn't actually "have" to, but I wanted to for a variety of reasons I can't be bothered justifying here :-D
It's like Christmas in here :)
Normally surgery on the engine bracket isn't needed, but I'm a compulsive Gizmologist. :-P
Again I couldn't help myself. As a result of the change to the engine bracket, I had to make my own frame strap with a wider reach between the holes. Used some standard builders strapping for this. Can't believe how many uses I've gotten out of this junk.
Engine mount plate is nice n snug.
My home made strap should be fine. (this is hardly high end engineering...)
A schwoit little 50cc four stroke engine with gearbox already fitted. 
You can see a little carbon on the exhaust heatshield plate. These engines are supposed to have been pre tested, with the carburetor idle and throttle settings already done. I'm really glad 'cus I hate setting that stuff up.
So I modified the engine plate, and forgot to add new mount holes for the engine itself. Doh gotta pull this off and fix that.
I fixed the engine hole problem, but realised I'd intersected too closely to the previous holes I'd drilled to make the plate fit the bike. THIS IS SO TYPICAL !! New hole... 
Engine sitting in the frame. Looks quite beastly in a pushbike to be honest.
Fitting the new rear sprocket to the rear hub. An interesting little press fitting design with rubber gaskets directly against the spokes right up against the hub. Very tidy.
I'll put some more pics up tomorrow when I finish the job. :)
Day 2 and more pics can be found here.
Day 3 (pedals only) can be found here:
