Gear attached to the motor – part of Sony X-3356-638-1 Motor (Reel R) Assembly. Gear is also compatible with Sony X-3371-429-1 that is actualy a substitute part.
X-3356-638-1 Motor (Reel R) Assy Gear 
X-3356-638-1 Replacement Part
Replacement part was created using a semi-professional 3D printer. Slighty flexible material guarantees silent operation. The part was fully tested.
The part is compatible with following devices:
TCKA2E, TCKA2ES, TCKA2ESU, TCK620, TCK850, TCK850ES, TCRX79ES, TCWR901ES, TCWR99ES.
It also might be part of other device – just check corresponding service manual.
Available in e-shop.
HI There
I have this exact part damaged on my Sony TC K611S and I was looking for a replacement as you can see in the thread bellow.
http://www.tapeheads.net/showthread.php?p=775428423&posted=1#post775428423
Perfection!
I used this gear for a Sony TC-WR901ES and works flawlessly and I will definitely use their products again…
I recommend Fixament!