The Holy Stone F181G Quadcopter is fairly tolerant of abuse (in my case, poor piloting). That said, the “motor wheels” or “small gears” have been the weakest link in my experience. In some crash situations, they tend to strip and no longer grip the motor shaft.
Holy Stone offers replacement motor wheels for other models, but at the time of writing, there are none for the F181G (besides the 4 spares it ships with).
Fear not, you can order an exact replacement on AliExpress for a VERY reasonable price:
The economy shipping takes a while (about a month and a half in my experience) but these gears work as perfect replacements in the F181G.
If it interests you, the specifications of small gears/motor wheels for the Holystone F181G are as follows:
Number of teeth: 9
Hole Diameter: 1mm
Gear Diameter: 3mm
Gear Length: 4mm
In a pinch, the F181G uses the same small gears or motor wheels as the SYMA X5C. The motors (diameter, 7 mm vs. 8.5 mm) are different, but the gears are the same. I ordered a set of Syma X5C motors on a whim to confirm this.
If you cannot find replacement motor wheels by searching for the Holystone drone, you may be able to find gears designed for the SYMA X5C which will also work on the F181G.