Secondary Quest Goodies:
Increased Nitrous Power - Beat all Racing Missions
Steel-Quilled Axe - Beat all Hunter Missions
Mouth of Metal -- Radio Player for The Deuce:
To find the Mouth of Metal upgrade when free roaming in The Deuce, find a spot with red flowers around a metallic relic/structure -- this is the entrance to Motorforge. The first one can be found just northeast of Bladehenge. Play the Relic Raiser and then enter. Enjoy the cut-scene and your prize.