I have thought of the same thing and even did the calculations to see what class you could run the car in.
Couple of good articles on Rallying.
Here are the adjusted displacment rules.
Most displacement limits are based on naturally aspirated, two-valves-per-cylinder piston engines. Anything more sophisticated has its displacement multiplied by whatever multipliers apply to it. These are the current SCCA multipliers:
Rotary engine: 1.8
Forced induction: 1.7
3 valves per cylinder: 1.1
4 valves per cylinder: 1.2
variable valve timing: 1.1
If an engine has more than one of these features, every multiplier must be used. An Acura RSX Type-*, for example, has a 1998cc engine with four valves per cylinder and variable valve timing, so its displacement has to be multiplied by 1.2 and again by 1.1. The total adjusted displacement is 2637cc
You have to use a NA 3800 or you will get moved out of Group 5 and you can't compete in the other classes with a 2WD car.
You would also have to convert the Bonne to manual since an automatic will not work in a rally race. You have to have a manual to do some of the rally driving techniques.
I think the Getrag 5 speed from a Fiero will work in a bonne and I am sure some other trannys will too.