#1 borrowed some bolt-on crank shorteners from John's Trikes and Bikes, and has had them installed on his Sprint FS-X E-trike for a while. They were set for a 150 mm crank length. That length works for him, so I ordered this crankset from Amazon: https://www.amazon.com/Vuelta-104-64-Crank-Black-175mm/dp/B005DUBLXC/ref=sr_1_1? dchild=1&keywords=Vuelta+MTB+Pro+44S%2F32S%2F22S+SQ+104-64%2FBCD+Crank+Set&qid=1583119912&s=sporting-goods&sr=1-1
 
Even though the crankset cost only $30, it's surprisingly of decent quality, and perfectly suitable for shortening from 175 mm to 150 mm. Thought a spare 118 mm bottom bracket I had in my spare parts stash would work, but it wasn't compatible with the trike's chain line (Too wide). #1 and I drove to RBM and purchased a compatible BB (107 mm). We completed the BB and shortened crankset installations this past Friday.
 
 

 
Tapping hole drilled in right crank arm.