Depends what they are made of, Danny. Wouldn't risk it, could make it too brittle, unless you have hardening experience and know exactly what your material is. Better to just buy a new one when its worn, considering the mileage and relative cost compared to bike.
Another possibility is to have a machine shop turn off the teeth and weld on a tooth ring, mean to do this one day with a pile of old worn sprockets I have lying about, relatively common * with old brit stuff.
*Relative being the operative word, used to know one guy that did it in the UK but it was cheaper to buy a new sprocket and chain.
As a matter of interest, local old time brit bike shop near here where I have occasionally been buying bike chain for 30 years has recently stopped selling Ren**d chain ( what I have always bought) and recommends IWIS ( German made) on the grounds that all of a sudden the standard has dropped massively on the first one. Purely anecdotal, No personal experience. On this. Chain quality itself is a minefield, when I first started working with George in 2007, one of his customers gave him a large roll of Renold chain the correct size, the guy had been putting it on bikes for years. Obviously didn't use them much, as it broke very quickly a couple of times one one of our bikes.. Turns out it was industrial, for slow moving machinery without much load, hence a lot cheaper than motorcycle chain.
Went in the dumpster so noone else could do the same thing.
