While the slope is quite obvious, I think you need something cooler, and more technological, to accelerate your Pinto.
You don't have a steam engine (and no fancy colored steam), but you have horses and oxen.
Those, though, are a bit on the slow side. But they are strong. Given enough horses or oxen, you will find yourself with plenty of power, but still not nearly enough speed.
Good thing you have the doc around, because he knows pretty well that there are nice technical ways of converting power to speed. One would be a gear box, the other would be a lever.
While I cannot think of a convenient and low-tech way of equipping an oxen with a gear box, I can think of levers, angular momentum, ropes.
And a spinning top.
So, we will build a centrifuge. In the center we have a drum with some length of rope wound about it, and on one conveniently long arm we have the pinto, and a counterweight on the opposite side.
Now, provided you can get a team of oxen to pull on the rope, you get the pinto spinning quite fast.
88mph tangential velocity should easily be achievable, and the moment you jump you are automatically released from the centrifuge (since it does not travel with you through time). You may want to choose a location where you will have enough space to brake, though.
The biggest advantage of this solution is that it is a lot more steampunk-hipster than just paving a slope!
Assuming a team of oxen will move at a speed of 2 km/h, and our Pinto is supposed to reach 141.6 km/h, the ratio of arm to drum is 70,8/1.
If we further assume the drum to have a diameter of half a meter, we get a radius of 25cm. Thus, the arm of the centrifuge needs to be 17.70 meters.
For a safety margin, make that 20 meters.
Or, you can reduce the drum diameter, or replace the oxen by horses. The much higher speed of the horses (assuming they should be able to reach 10 km/h) even when pulling this weight), we get the length for the arm at 5 meters when the drums radius is still 25cm.
All in all, I would say this is completely feasible.