When they banned forced induction, some people put N/A 2.5 flat fours in their Subaru's. Similar power, much more noise but totally legal.
I've also done events on a navigation permit when someone entered (legally) a Mitzi Evo VII and we walked all over him in a standard 1.3 106 Rallye until we lost our brakes (I've also won a 12-car by 6 minutes in a standard 1.8 Vectra and beat some serious MkII's etc). Having a quick car with a rubbish navigator will just get you lost quicker.
Most events can be won by a good crew in a sub-1600 modern car, but I can't see the logic of banning totally standard cars that were built with twin-cams, twin-40's or 6 pots. People aren't going to start using Gp4 RS1600's, Escort Twin-cams or RS1800's on road rallies anyway.
A sorted 2.1 Pinto on bike carbs or a decent red-top in an Escort is still a formidable car in the lanes in the right hands.
If I was sitting on the MSA board, I'd propose a 1600cc limit on 'modern cars' (say post 1980) - pre 80's let anything that was standard run with full induction filtering (standard airbox etc) standard exhaust, standard brakes and standard gearbox/LSD.
Ban moulded slicks, road pattern tyres only - only allow cage/seats and belts, underbody guards, brake material and shocks and springs. Hold on, that's almost Enduro regs
We can only dream !