There would not have been any 1971 cars with 2.5's in 1971. However homologation for those (actually all) cars goes well beyond the one year and these cars, and the homologation papers were continually updated. So I would have thought by 1972 and 73 there would be a number of 71 MY cars "upgraded" with 2.5's A good example is ex werks 1969 rallye car 119300932. That car was sold to various French owners post its period as a factory car and was entered in a number of Le Mans 24 hour events. In 1972 it was running a 2.5 engine as car #45.
So if you google the time it was sold a year or so ago all of that history will be there. In addition the Le Mans entry documents and race records for 1972 will show it as #45 with a 2.5 but it is a 69 shell and using, I presume, updated 1969S FIA papers to compete.

I would have thought there were similar records of 1971 cars but I cant see why you cant use the fact that the FIA papers which would have applied for 71 cars (actually a 1970 FIA document i think) were continually upgraded to keep the cars competitive

I would have thought that a conversation with whomever would need to certify the car before proceeding too far would help you in this quest.