Keep in mind that you are permanently altering a part that will not be sold to you by the manufacturer. Here is what I do:
I have the proper jig to hold the bolt in place in the lathe while the knob is turned to a stump and threaded. I then make a knob to my personal preference from a piece of steel. Scrapped rifle barrels are a useful source.
I have also used a hacksaw and files to reduce the bolt knob area to a cylinder, and threaded it. If your DIY skills are good, this is definitely an option. If they aren't, a badly damaged bolt handle could probably be cleaned up with a jig in a lathe.
The black plastic knobs sold for machinery handles are dirt cheap, and make a nice oversized knob - although they do not extend the length.
A variety of commercial knobs are available.
There is even one plastic knob that clamps over the existing knob.
Another option is to replace the entire handle. Remington handles are induction brazed in place. A replacement handle can be silver brazed to the bolt body. Don't even consider doing this without the necessary oxy-acetylene torch, jig and skill set. I have heard of handles being TIG welded in place. I have no experience with this method. If the entire handle is going to be replaced, I prefer the Holland handle, which can be obtained from Brownells.
In the photo is a Remington handle (older version with narrow shank) altered using the jig, and fitted with a knob I made, and a Holland bolt handle silver brazed to a bolt body.
