Vincent:
I had two bikes of this design from different manufacturers. One had the issue you mention. The entire bike wasn't of the best quality. But it was fast, did 22 mph, with a 250 watt hub motor. That bike was stolen. The one I have has a 350 watt motor with far better components. All in all, the 250 watt bike was a better combination of the important things (even had a cruise control) but a collection of cheap parts and poor fabrication. Below is a video showing that bike.
Note that this bike had gears on the crank. I replaced them with road bike gearing (as shown in the video). Made it a much better bike, it was geared way too low and there was much duplication in gearing.
Remember the "Hidden Cash" guy? He hid money and left clues as the where it could be found. I saw the clues and knew where the event would be. I had the lock, but didn't have the combination, so I couldn't leave the bike, but could see some of the plastic "eggs" that were hidden from up on that pier. It was genius marketing, for $800 he had a few thousand people out enjoying themselves. If someone found $20 it made their day, if they found nothing they still thanked him for inviting them to the party.
Those triangular parts on the bike in the video bike were just pieces of aluminum sawed out with holes drilled in them. The whole thing got looser as the bike was used. I believe the bike I have has bushings in the holes, maybe just better size holes.
Pull one of those pieces and see if it has bushings. If they are the wrong size or just drilled holes, it's a very easy thing to bush the holes and use high tolerance bolts to put it all together.
If this is a bike you like, you might look at all of the holes. Bushing holes is basic machine shop stuff. My guess is that it might be money well spent.
I import bikes (and other things too), manufacturers send me bikes to look at. I may be able to get you any parts you need (I know the right guy to ask). That bike has a very good manufacturers warranty, I'm sure all of that is covered. Ask the dealer you bought it from about the warranty and then look into the tolerance of the bolts and holes.
Good luck!