I am new to on-road nitro and I am wondering what mods if any are allowed under ROAR rules? Engine tuners claim increaded horse power, more consistant power curve, and increased fuel mileage with their mods, but is it leagal at sanctioned events

Yes, it is legal. They change the timing in the crank, bore the hole in the center of the cranks larger, balance everything. Put silicone ramps inside to smooth out the fuel path into the crank. Knife edge the rods. Cut teardrops at the inlet ports on the sleeves to help the fuel mixture into the cylinder. Better and lighter bearings.