The group 3 cars used 1800cc 8 valve motor with narrow wheel arch extensions, Group 4 cars had 1800 engine with Abarth 16v head as used on Fiat 124 and were good for 190BHP @ 7700rpm.
There have been 3 types of 16v head used on Twin cams, the early Abarth 124 type (as used on the 124 & Beta), a later Lampredi design as used on Fiat 131 16v, lancia 037 etc, and finally the Fiat/Lancia production 16v as used on Delta Integrale, Fiat coupe 16v, Fiat tipo 16v, lancia Thema 16v & Turbo etc, etc. This is the head that is reversed compared to early type. There was a Guy on the Guy Croft forum who actually managed to reverse the camshafts in the later 16v head so it could be used back to front, so it is not impossible
Have attached a picture of Beta Group 4 16v