The first thing I'd do is contact Tern and see if they can supply a replacement. That's the easier way to make sure you get a compatible crank.
I don't think it's possible to buy Prowheel components directly (at least they didn't show up in a couple of online bike retailers I tried) so you need to find a compatible replacement. From the Tern Verge D9 web page we can see that the crank is single ring, 53 teeth and fits on a cartridge type bottom bracket - presumably with a square taper axle-crank interface. The info written on the crank tells us it has 170mm crank arms (measured crank spindle center to pedal spindle center). The tooth count and crank arm length you can of course verify.
You can search for folding bike cranks that match these specs. An alternative is to look for 'track cranks' (used on single speed velodrome track bikes). These may be easier to find but typically have smaller chainrings, but you can also find replacement chainrings that match the bolt pattern and circle diameter.
The complication is that each crank model requires a specific bottom bracket axle length to get a specific chainline - how far away from the center line the bike frame the chainring is. It can be tricky to find this info for cheaper and non mainstream brand cranks. You may need to replace the bottom bracket with one with a different axle length to get the right chainline.
Special tools are required to get the crank arms off the axle tapers and to remove and install a cartridge bottom bracket from the frame. Additionally everything really need to be installed with a torque wrench to make sure bolts are tightened to proper specifications.
On that subject, threads in the crank arms can get deformed and ruined if the pedal spindles are not tightened properly. The hard steel starts to wallow around in the softer alloy crank threads. That's probably what happened to you cranks.
If you are not familiar with bike repair (presumably you are not, as you're asking here) you'll need to enlist a local bike repair shop to help you install the crank, and they will of course be able to help you source a compatible crank.