Motorised bike (honda GX, electric?)

has anyone here attempted to put a small petrol engine or electric motor on a bicycle? or build a pusher trailer for a bike? I was going to try something like this guy made - looks easy enough for a n00b since I never tried anything like this before http://www.youtube.com/watch?v=zpjh2EfzqhA

will probably be a while before i do it, i will be having a look at the car boot sales for cheap engines since I dont really feel like spending 200e+ on a GX25 right now. the roller type set up in the video looks like the easiest to make because you dont need reduction gears and the like.

back in '04 i used to have a bike with a small petrol engine on it but it was a commercially built one and had a top speed of 18-25km without pedaling which was alright most of the time but i felt like it could do with a small bit more power. around 30-40km would be grand like cause that yoke i had wouldnt have been much good for going uphill