The rules are here.....
www.nasaproracing.com/rules/944_spec_rules.pdf
4 Eligible Models
1983-1988 Porsche 944, Normally Aspirated, 2479 cc, eight valve engine
1987-1988 Porsche 924S, 2479 cc, eight valve engine
4.1 Chassis Swaps
A 944 spec may utilize any year chassis in the above eligible models as well as the 1987-1988 944S and 1989 944. No other 944 or 924 type chassis may be used to build up a 944 spec. In all cases body work, engine, transmission, engine electrical and suspension components must conform to the list of eligible models and to the modifications expressly listed in these rules.
There must be some difference besides the motor. Bigger clutch, stronger transaxle, different gear ratios...............