Nope i am afraid that there are no floor repair panels avalible for an uno in the uk that i am aware of,or anywhere else to my knowledge.
I had this very same issue when i brought a very rotten mk1 turbo,and in the end i just made complete new floors out of thicker 1.6mm steel sheet.
Imo the flat 1.6mm floors more than matches the strength of the original 0.9-1mm fiat floors, but if you are worried about strength maybe think about getting them swaged like the originals.
Anything is repairable and being a nearly extinct uno turbo i would say it is definatally worth it,but if you are not carefull it can turn into a never ending epic battle to completly weld up and seal an uno, as more and more rot holes will appear with a bit of prodding lol.
Also as its for you son you really want to make sure its all done 100% right and good and strong,as these things really dont take a crash well even when in good condition, and if rotten or bodged they are a death trap.
The other option is to reshell it,this is easyer said than done though as finding a genuinely solid uno nowadays is VERY VERY hard indeed, and if not extremely carefull you can end up straight back to square one again with a rotter.
The work involved in reshelling is by far alot easyer than welding/repairng/painting a complete rotter though,you need to assess the car completly before making a decision imo,but as mentioned it is very hard to find a good shell to reshell into so be carefull.
I am giving both ways a try lol and atm am nearing completion of a reshell.
I basicly finished welding mine up then started to get slowed up by the body work prep, as its not really my thing bodywork and filler etc.
Anyhow i luckerly found a very soild mk1 45 to reshell into,and after spending months of welding one up this other car was just to clean to turn down.
Here have a look through my build thread, the first rotten car with the floor is at the start and the reshell at the end of the thread.
You could even maybe join the forum?they are all really a great bunch of enthusiasts and are more than happy to help
http://www.turbocollective.net/phpBB3/viewtopic.php?f=49&t=3261&sid=77d3dc99a0745eca762f680808101646
