There isn't realy any extra stress because the motor is in the back wheel. The rear triangle will easily be strong enough and the torque from the motor only reduces the stress on the folding mechanism. I thought you'd have figured that out.The Apollo Transition bike is a machine I know quite well, simple inexpensive and perfect for nipping round Tesco's and picking up the kid's from school, conversion to an e-bike, well that's a different matter.
The mechanical stresses on a folding bike are very different to a traditional diamond frame for starters, where would you say the weak point is on a folder...the hinge ? Folding bike enthusiasts reading this will know it's not so I wont go into the stress dynamics of the frame regarding shearing and torsional forces.
The Transition is a bike built to a price point...cheap, hence my reference to it's cost. As such it flexes creaks and groans which makes it entirely unsuitable for converting to an e-bike bearing in mind the extra stresses it will encounter.
Could it be done ? yes easily I'd say, I believe someone fitted a ram-jet to one once ! but IMHO the answer is don't.
I'll look forward to the 1000m review