I can take carry on the bus will take me farther
I put a motor on a Tern S8i, but used a front hub motor.
Tern makes ridable bikes, and here in Perth a folding bicycle may be taken on any bus or municipal train (Transperth), although it must be covered so other passengers aren't concerned about greasy chains.
I have moved to a trike now, and sold the Tern. If I were to attempt this again, I would look at several possibilities:
Folder with rear hub motor and derailleur. That compact wheel makes large sprocket ranges problematic (derailleur cage length hanging down), but with a motor I would not need a large range. I would aim for a low that will get me over hills on the way home with a flat battery or other problem, and not worry about high speed so much. I like having teeth.
An Efneo 3 speed planetary crankset can multiply the rear cassette as well - I use one on my trike now, and it's nice.
Folder with a mid-drive. This seems like an excellent combination, but the big issue here is getting the chain line correct. Folders have short chain stays which exaggerates any mismatch. A folder with a belt drive (Tern S8i) has already determined the chain line, and the mid-drive must accommodate that. Praise if you find a working solution - publish it here so others can do it.
What I did - a front hub motor. The Tern S8i offers a full 100mm front fork, so many motors were available - I used a Grin All-Axle and it worked quite well. Many folders have narrower 75mm forks and Grin carries various kits using the Crystalyte SAW for this:
Grin SAW kits
The All-Axle could apply enough sudden torque to spin the front tire on uphills if I was injudicious. A smaller motor may lessen that problem. I put an All-Axle on the rear of my Cruzbike Q45 (they are built backwards) and never had that problem.
A 4kg battery and a 4kg motor on a folder is a big deal. Lighter components on a folder for around-town bus & train use makes sense. The larger and heavier arrangement makes sense on a folder for long distance travel - the Tern P27H was a 27 speed touring folder with disk brakes. The gearing was all in the rear hub with a 3-speed internal and a 9 sprocket cassette. If I were doing that, I would use a rear hub motor with a 9 sprocket cassette and an Efneo crankset. For touring, the All-Axle, as it has fewer failure options.
And I would wax the chain.