In principle, solve these parts.
1st FSM: Axis to Rotation
Get horizontal axis and use the value (which goes from -1 to +1) to incrementally turn/rotate the object. Observe the scene view and see the arrows when having the object selected. They represent the forward vectors. Make sure your bike is aligned to the correct axis (or better, try with a clean scene and prototype there with a sphere, add a small coloured cube with no collider as a child, so you see instantly where forward is).
2nd FSM: Speed
Get vertical axis and store the value. Usually, float multiply it with 5 (as a start) and store that on a new variable, and use that variable to set velocity. Make sure you do all of this in one state and every frame.
This outline should work. Problems with slopes and such should disappear when you set the rigidbody collision as continuous, and by using a sphere collidere, where you can freeze their rotation. The colliders should be used in a functional way, not mimick how your object looks like, and you can easily use multiple on the same object. Use a sphere for the base (or where the wheels are) and add cube(s) offset above when you want collison there, too, but make sure that the base (where it drives) is round.
Controls should be easy to set up and get going, but they require A LOT of time to get right. You need to tweak gravity scale on the object, or set up more elaborate ways how velocity is added etc.