For anyone interested in converting your bike, I found the exact answer
I was looking for he http://www.motorizedfoldupbikes.com/

It's a US company who makes simple engine kits to install onto the back
on a bicycle. Quick release, so I'm not sure how secure that would be
in public.

If anyone would care to look at the website and comment about the
different models of engine available (they have five engines named
under the ENGINE KITS menu on the left). Unfortunately the people who
made the website decided to give a specification for each engine, but
different statistical information for each one, so its harder to
compare some of the models.

Prices are pretty good though, around $300 to $450 - much better than
buying a complete petrol bicycle all in one.