Yes, a motor + battery kit. It's really simple -- depending on the bike. For a folding bike you need to think ahead of where the engine and battery are going to go when folded. Mid-drive motors are much preferable, if possible (at the center of the bike, between the cranks).

There are two Chinese brands for the engines: Bafang and Tongsheng. Bafang has a better reputation; I used a Bafang for my first conversion (a regular MTB). But Tongshengs, in addition to being a little more affordable, are true pedal assist: power is proportional to the force you put on the pedals. It's a different experience from Bafang, but pedal assist feels better on a small folding bike, IMHO.

You can buy them on Aliexpress; look for vendors who have inventory in your country, which allows for faster shipping.

Here's the final build:

https://i.imgur.com/TBHHOc1.jpeg

... and on the road in Marseilles:

https://imgur.com/a/B2mJ17E.jpeg

Look into velogical motors. Their tiny build size and removable battery should be a perfect fit.

http://www.velogical-engineering.com/velospeeder/produktinfo...