Hi Wes, Let me add what little knowledge I have on this subject. I enjoy fabrication, and "thumb nail engineering".
The Schwinn Town and Country three wheeler in the photo is a normal middleweight Schwinn girls bicycle with a Triwheeler conversion kit added. The conversion kits were made by Ret Bar Engineering in El Mirage, AZ. Remember this is all 1980's information, I'm sure much has changed in the past forty years. Ret Bar shipped the raw parts to Schwinn Chicago, Schwinn built the bike and shipped it back for sale in Arizona Schwinn Dealerships. Over the years there was two different style kits, a single wheel drive kit, and the bike pictured has the dual wheel (differential) drive. In our Arizona dealership which was close to the Retirement Sun City tribikes were a popular product. We bought the parts directly from Ret Bar and converted our own bikes, for a large freight savings.
Adding any kind of an assist motor gas or electric IMO would be marginal using the original kit drive parts. The differential is not lubricated and will not sustain any higher RPM speeds. The rear axle bearing retainers are simple press-in (cheap) items. They were marginal for slow speed use, they are not designed for any extra torque or speeds.
As a fabricator, you can engineer most anything to work. But it would be a full re-engineering on the drivetrain. Good luck with your project.
John