Perfect! Next recomendations:
"Health Bar has a red trim around it " - this is hardly possible to fix without interfering with the settings, so the ideal solution would be to add two new sprites that should hide the red frame. (Draw a sprite that will be empty inside (Frame) and place it on top above the “Life Meter”)
“and goes down super fast does not give a chance to play the game with all of the power ups.” - The fact is that the Unity handles the event faster in which your opponent comes into contact with the player. Add an additional condition to the event to do this, for example:
Player Car/Sensing/Is Touch Object/Enemy Car
System/Runtime/Repeat Every/0.1 sec
Enemy Life/Life Meter/Subtract/1
“Tap to start appears on the screen when the player presses “Play Again Button”.” - as far as I remember the your project, this same result we have in the “Ready Maker”. This hapines because you restarted the all project. (Check events in the Ready)
“You’ll see some black textures from out of world.” - This is due to the fact that you use the phone screen size in the your project (Landscape/Phone mode 10:6), when you create an apk file, your scene should be displayed correctly on all mobile devices.