Scuby engines fit take a look at Z Cars, a car mod shop in Britain, they have fitted a couple to the original fiat 500. They have mated a Brat 1800cc to a 126 gearbox and installed in a 500, they also used a 2000 or 2500 turbo with a modified scuby box, mod parts from Australia and installed in a 500. There are build videos on the net, can't remember where though. I have seen a rotary in aTriumph Herald, looks as tough it would fit in a 126 with little modification.
Ralph