Look at some of the Fiat 500 stores:

Essentially the 126 has an upgraded Fiat 500 drivetrain and is frequently
swapped into the 500.  

Here are a couple of additional links:

http://www.fiat500sport.com/
http://www.fiat-500-center.de/FRAME_GB.HTM

For a relatively cheap dual-port head, you can use the Fiat Panda head.
There are many manifolds available to adapt it to side-draft double
barrel carbs.

You should be able to find up to 800 cc cylinder kits.
Also, 5, or even 6 speed transmission kits.

I know there are also rack and pinion steering kits for the 500 (and I
presume 126).

Have Fun,
Clifford