I haven't heard of Black Horse before, but a Cub is a Cub. Measure the wing chord and put the CG at 25-26% back from the leading edge. 71" will be a smallish .40 size Cub, so any motor intended to substitute for a .40-.46 will do fine. Pick the lower Kv choice within that range so you can use the bigger props since Cubs are so draggy. You can get amp draw data once you've chosen a motor and a prop in order to buy the right batteries.