The displacement of a single rotor in a 13b Mazda is .654 liters, so 12 of those would be 7.85 liters, or 15.7 liters in 4-stroke piston engine equivalent. The last Wankel that Mazda produced was advertised at ~238HP @ 8500 rpm, so if they can duplicate that specific output, they could get 1428 HP.
Supercharged big-block Chevies that put out ~1400 HP for offshore power boats weigh about 1000-1100 pounds, so the 12-rotor Mazda would be competitive on weight.
The 4-rotor Mazda engine that won LeMans in 1991 put out ~700HP, so power would scale to 2100 HP for 12 rotors in full racing tune.