Several rowing bicycles are available aiming to improve upper body training. However, they are expensive, it is difficult to find local distributors and most of them are not really suitable for everyday use. This one is cheap, easy to drive and fulfils all traffic regulations that I’m aware of.
I open a blog for Spanish and other languages speakers: http://lamarianette.blogspot.com/
You may also check my second instructable on this subject, explaining some improvements over the original construction: "marianette update".
To make place for the rear hub in the front fork, you may deform it using a dismounted hub as in the picture:
Fix a pedal on the stem or handlebar and link a bar from that pedal to one of the front pedals. You will have a crank mechanism: http://en.wikipedia.org/wiki/Crank_(mechanism)