Ah so you're looking for acceleration?
If so then you could use a float to represent the position of the motor between the minimum speed and the maximum and use that as a multiplier for the actual power output. So between 0 - 1 like you're saying.
In the input settings you can define the rate of increase/decrease for an input axis to somewhat control acceleration and use Get Axis to grab it, but this method won't scale with the project very well if you intend to control acceleration more precisely.
I haven't considered Acceleration much, really. It's something I'll get into soon.