Hi. My name is*****'m a camera repair technician with almost 30 years in the business and I'll be glad to help you with your question today.
The lens mount can be replaced fairly inexpensively. If you do it yourself you will need a small, Phillips Head screwdriver and remove all the screwa you can see on the mount, including the ones around the lip. be careful not to tear the flex cable on the contacts. You may also need to unsolder a black wire on the contact you need to transfer to the new mount. You also need to transfer the diaphram control linkage from the old mount to the new one. It's not very complicated so there isn't much that can go wrong. Or, you can usually get it replaced for you at a shop for around $50.
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Hi. There isn't a part number for this part as Nikon no longer sells them. However, it called a bayonet mount and is available from many sources on EBay.
As for where to get it repaired, you may want to check locally for a camera repair shop in your area. I'll be happy to help you find a place to get it repaired. Perhaps this would be better served with a phone call?