Hey man, i got an answer of the same Race 504 i sent u pics before. So there are the specs if car respect´s the regulations of "DTNH" - Desafia Turismo Nacional Historico.

Max CC: 2000
Max Intake/Carbs: Dual 45 (Single Carb)
Spark Addons: Allowed 6AL
Max Power: ~ 145HP (Regulations has 10% gap after race. So they can reach 150/155HP without penalty).
Suspension Geometry: Standar
Suspension Springs and Shocks: Free
Differential: Auto Lock
Tyre´s: 195 - 60 - 14" (Street compound)
Brake power: Standar discs with soft pads allower
Minimun Weight: 1050kg (dry weight)
Ballast position: Free (Including fuel tank and battery position).
Seatbelts: 5 points

Those are the most relevant parts of the regulations for the car. Hope they help! Most of those 504 are standar 2000cc Pugs that are called "504 TN" they came out from factory in Argentina with 112HP to wheel and wider rear rims.

That could be a very intresting version that u could do. The difference´s are the Twin light front and the rims 6" front with Good-Year G-800 "Grand Prix" 185x14 and 6.5" rear with Good-Year G-800 "Grand Prix" de medida 195x14.
Here´s a test review, the most important things are easy to read even in spanish, but if u want more info i could translate it for u man.

Oh, an important info: The tire´s are 185x14x15" thats an old way to messure the tire´s. its almost a 185-60-15". The race versións of 504 use 14" most cases or 15" with 185-55-15" tyres (wich are better but expensivier).

Some important things about TN u could not easly see in the review:
camshaft 7.5mm instead 6.7mm
engine ventilation with clutch (u could cut the pulley from alternator and other peripherals with ~5hp gain)
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Differential final ration 9x35

Suspensions 30" less travel / 15% stiffer springs

No sunruf and 20kg lighter.