Updated: Feb 9, 2017
The 903 is still in process, but I wanted to share with you all what I am up to.
In order to install the 903cc engine from a Fiat 850 into a Fiat 600 you have to reverse the rotation of the bottom end. The top ends rotate the same direction. Chris Obert wrote a pretty good guide. The easiest way, I think, is to replace the timing chain with timing gears. The gears can be purchased from Exotic+Raceparts here or from Fiat Plus here. You will also need a spacer because the timing gears are thinner than the chain sprockets, which you can have a machine shop make or purchase here. Also, you’ll need to replace the front and rear main seals with non-directional or clockwise seals.