Mike has trike experience too. I had motors too fast for them, meaning the adult trike, delta kind trike. Worksman type.
So what I found was that a Schwinn trike gets hard to steer at more than 15 mph, very hard to jink around the glass in the bike lanes. They also pull like hell if you ride on a tilted shoulder, at 30 mph, it will throw you into the ditch. Apparently the tadpole trikes don't do that.
But at lower speeds, the trikes can rock. I used two of the schwinns at work for years. I was head of maintenance at a very narrow street condo complex. A truck for tools on the street sucked, what I needed was a tiny pickup 3' wide. The trikes were perfect. I would tow garden wagons, sometimes even two of them at once like a train. 5mph stuff. But over time, even at 5 mph, the speed bumps in the complex would trash the back wheels, and even bend the axles. We were never loaded less than 100 pounds in the basket.
The ideal motor for a trike is E-Bikekit's trike front motor kit. It's max speed is about 12 mph. Perfect for the fixed gear of the trikes too, if you don't have the three speed.
But if you are a bit braver, it can also be fun to ride them faster. Mastering the two wheel turn at 20 mph is fun. A nice setup could be a mac 12t, and three speed switch. I'm not sure how the three speed hubs on trikes would stand up to a bb motor.