To me it looks like the cross section of the new green belts is too small and may possess the wrong angle.
The Gates Fleetrunner Belts are said to be "used on specific lawn and garden applications, vintage vehicles and specialized applications". These belts only come in widths of 13mm and have an included angle of 38º
This might not be the place to look for a belt to fit your main engine.
You noted in post #12 that you tried to match the size of the existing belts, but may not have been able to do so. I think you are right, they don't match.
You may want to consider doing something like this.
Remove all the belts and using a drill motor mounted wire wheel, clean up the pulley grooves until they shine. The unused pulley groove in picture #3 looks mean.
Run the vacuum all the time and spent some time cleaning up the iron dust.
Hopefully the engine manufacturer can tell you what the pulley spec's are so you could find the right belt. If not, I would measure the width of the groove at the top, which from the pictures looks to be wider than the 13mm width of the green belts. Then from the Gates Truck Belt selection find a 15 mm (0.59") wide, 40º belt of the right length and test the fit. The belt should be just proud or flush with the pulley.
Gates Truck Belts are specifically designed for heavy-duty truck, bus and off-road equipment applications.
www.gates.com
Similar sized belts are offered by Dayco, Goodyear and others.
While you are at it, spend some time checking and adjusting the pulley's alignment, which to do correctly will take a bit of thought and effort.
Once done, belt dust should disappear.