As an inveterate putterer, I'd try a shim.

[On one of my banjos the nut slots looked a bit uneven, so I set a nail that looked to be an appropriate diameter right in front of the nut. It looks better.   ]

I have shimmed the nut in one banjo and used the superglue and baking soda technique to fill then refile the slots in two others. All these were intended as temporary fixes, but I never got around to buying or making new bone nuts.

On one low-end banjo I lucked out with a ready-made pre-slotted bone nut that was a drop-in replacement for the existing plastic nut. There's a chance it could happen for you. But no guarantee.

I think my only point here is that you might want to do the quick fix so you're not without the banjo. And you might want to do it with the intent to do something "better" for the permanent fix. But if the banjo plays well and sounds good with the quick fix . . .

I have made a lot of guitar nuts and banjo nuts over the years.
I maintain that If the existing nut is in good shape and the nut slots are still crisp and serviceable but one slot has been cut too deep, shim it.
If the nut slots are totally worn out on the existing nut and need to be recut then it’s best to build a new nut.
When shimming use the same material as the nut and cut it a bit bigger. Super Glue the two together then lightly sand the shim to the size of the nut (or nut mounting slot if the original nut has a sloppy fit) making sure the nut is flush with the fingerboard.

If the head is slack, it could cause that G string to buzz. Perhaps a new or stiffer string. banjered

If the nut will slip out easily, a shim can be a quick and easy fix. I have had good results with white super glue and bone dust to fill and refile nut slots

Have your dentist do a filling of the slot that is worn too low.

I've turned a nut upside down and slotted what used to be the bottom.
This depends on if there is enough height left after sufficiently flattening what used to be the top.
I've also used shims under banjo and guitar nuts with good results.

I like using rosewood under nuts,but anything should work.

I've never had a problem with a banjo nut, so I've never shimmed nor replaced. But I had an NICE OLD Martin guitar with a homemade shim nut and it irritated me so bad to see it, that I had a pro replace it. He did a capital job and IT IMPROVED THE TONE! Solid connective between a high quality animal derived nut and a totally flat peghead surface DOES matter for tone.

But I agree with everyone else, try a shim to see if it solves your buzz problem, before you invest in a proper nut replacement job.

Get the BEST quality animal-derived nut you can find and afford. Pre-war, Gibson's best models used mother of pearl for the nut. Harder than a whore's heart, as my relatives in Maine would say. Go for TUSQ if you have to.

The old women in the service department are waiting for ye noo in heaven.

I did the beef shinbone with help from my Jack Russel terrier and Az. Sunshine, then I learned about bone dust and a mask.

Superglue is not my friend, just a neighbor.

I've used TUSQ guitar saddles and nuts and like them better than bone (except for the price).

After getting tired of the "squeak" related to bone nuts as the string wore into the slot I went to pearl banjo nuts in the 80s (StewMac) and never looked back.

Pearl is much harder than bone and is for all intents and purposes immune from strings wearing down into the bottom of the slots and I've heard people say that pearl is too bright,but I haven't found that to be the case.My flatheads w/pearl nuts have great bottom end "growl".

I believe JD Crowe used pearl nuts and his tone has never been described as bright.

I have done this quite a number of times on various banjos. Shimming is the easiest approach. Nut slots can be filled and recut by the method mentioned by Ken (Old Hickory). It is whatever works the best.