If the motor on the kit you returned was designed for a bigger fork, then I don't understand how washers would male it fit on a bike like the one below.
Let's see a link to that kit you mentioned to see if the dropout requirements are mentioned/ .
And while you're at it, what Workman trike is it?. Is it one of these? A 2.3 inch tire on the motor kit probably has a fairly wide rim. The bike below shows caliper brakes, and you might have to hunt up wider brake calipers to go around a 2.3 inch tire.
https://www.worksmancycles.com/pt2f.html