What's best engine to motorize a road bike
There are some bike hubs containing an electrical motor inside. If you google "bike hub motor" and take a look at the images, you'll get the idea.

I think these ones are ideal since they require minimal changes to the overall bike structure, allowing for normal riding if the motor is not working, and they don't burn evil oil: you just plug the bike to the wall and that's it.

The spoked ones should be adequate for building a front wheel with a regular road bike rim. 

Hope this helps