The gear is part of the Sony Minidisc MDS-JA30ES and MDS-JA50ES tray assembly. Sony part number is 4-987-293-01 Gear (4).
Replacement gear 
Replacement gear 
Replacement gear in 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 Sony Minidisc Recorders MDS-JA30ES and MDS-JA50ES. It can be compatible also with other devices – just check your service manual.
The gear is available in the e-shop.
X33635011