The gear is part of Sony Minidisc MDM-3A mechanism – optical pickup KMS260A and KMS260E. It drives screw that positions the laser pickup. Sony part number A-3304-200-A.
Replacement gear 
Replacement and original gear 
Replacement gear in the assembly
Replacement part was created using a semi-professional 3D printer using durable Nylon material. The gear was tested for correct operation.
You can find the gear here in schematics:
Gear is compatible with following Sony Minidisc players:
MDS-JE500, MDS-JE510, MDS-JE520, MDS-JE480.
However, it might work also with other devices – just check your service manual.
The gear is available in e-shop.
(plastic gear only, without the screw rod)
X33635011