Assuming an average horse weights 1,500 lbs and produces 1hp. A 50hp motor would need 75,000 lbs of 'horse'.
Assuming an average Eastern Grey Squirrel weights 1lb, and assuming one pound of squirrel produces as much work as one pound of house, that would require 75,000lbs of squiril or 75,000 average Eastern Grey Squirrels to make 50hp.
You are looking at thousands of SP (squirrel power) with your build. congrats!
Disclaimer: none of that is accurate.