someone put a 6 cylinder Dino engine in a Alfa Spider years ago and used to bring it to the local shows. called it an "Alferrari"
over the years, many people have done engine swaps using the Fiat twin cam engines in other cars, but i have never seen a really good swap done on a 124. the V8 swaps are a joke and simply ruin the car with all that weight on the front end.
Since you can fit a 2 liter Fiat engine in any of the spiders by using the later big bump hood, and you can make 140+ crank HP those engines and some tuning without ruining the street drive-ability. I make 130 HP (crank not wheels) in my street tuned 1800, and that engine makes 100HP already at 3000 RPM's. with 130 -150 HP and not too crazy of a cam, the Fiat 124's are actually pretty fast and can keep up with more modern sports cars. no big time engine swap needed.
1967 Fiat 124 Spider
1964 Fiat Abarth 850TC conversion
1962 Abarth Allemano 1 liter Coupe