How powerful would this motor have to be?
With your limited information it's only possible to give an lower limit of the mechnical power needed (i.e. more is needed because you probably also want to accelerate your load and you will have some friction; both are not considered because of missing information; also since this is only mechanical power you still need to consider that the efficency of the electrical motor is less than 100%; so its electrical power needs to me more):

5 MPH = 5 * 1600m / 3600s = ca. 2.2 m/s

Since inclination is 20% you have 0.2 * 2.2 m/s = 0.44 m/s (height per time).
Power is work per time and work is force times length.
So Power is force times speed.
Force in this case is is weight; which is mass times gravitational acceleration (on earth ca. 9.81 m/s² = 9.81 N/kg)

P = 100 kg * 9.81 N/kg * 0.44 m/s = 981 * 0.44 Nm/s = 431 W