There's 3 things that hold the motor in/to the gun (from the bottom to the top):
1. Grip plate (aka Heatsink)...the metal cap that covers the bottom of the grip, held on by 2 screws. There's a central screw/allen key grub screw in the middle to adjust the motor height...this adjustment screw either pushes directly against the motor's end cap or against a small metal disk (is that what you're missing?).
2. Grip...umm, it's the grip
3. 2-4 screws that hold the grip to the mechbox.
If you're missing the small metal disk...you can cut one out yourself from sheet metal...just match it to the size of the end of the motor cap. Or check airsoftparts.ca...Jugglez usually has some in stock.
Note:...with some grips the adjustment screw is really large and a 3mm ball bearing is used...it's positioned between the adjustment screw and the axle of the motor.
...I've seen plenty of grip caps that have a large adjustment screw...and nothing else inbetween the screw and the motor. Doesn't seem to matter that there isn't a disk/ball bearing present.