Steyr Puch didn`t customise 500`s they had the rights to manufacture them, but being a manufacturer ( Austrian) amongst other things in their own right chose to use their own engine. They are a military vehicle and equiptment manufacturer, who at the time they were making 500`s also made mopeds (Puch).
The Fiat Panda 4wd and Alfa Romeo 33 Series 1 4wd wagons used a Steyr Puch system under licence.
The New Zealand Army and other Armies use the Steyr semi auto rifles currently, plus run their troop transport vehicles.
The Steyr 500`s I think from memory ran a flat engine compared to the inline Fiat unit didn`t they?
Interesting wee car though but I wouldn`t think that rare in mainland Europe, is it?
Richard Jones