Don't despair at some of the comments, as far as I am concerned it doesn't matter how much you modify the car by. If it's only minor cutting lumps out to smooth the body or fit a more powerful engine it's all up to you. There are a few 126's with fire engines fitted and running in Britain not to mention the Boss brothers with their v8 powered, mr 2 powered and some other engine in a third and all road legal. There are several fiat 500's with Subaru engines even some with the subagears gearbox conversion to allow all that power to grip the road. Admittedly I have only seen road legal fire engine conversion on this site. It's up to you and I can't wait to follow your build. If I can find a builder to build my garage I will be able to continue with my Subaru conversion, I have the engine mated to a bis gearbox ready for a trial fit, but as I say it won't go any further until I have a dry environment to work in. All the best.
Ralph