I'd clean the theads with a 1/4 inch NPT tap and then screw a 1/4 inch NPT nipple (with plenty of teflon tape on it) and a 1/4 inch NPT ball valve into what you have.

From then on, you can simply connect to the brand new brass treads on the 1/4 inch ball valve. And, if those threads eventually wear out in time, just replace the 1/4 inch ball valve with a new one.

You should not have any problem finding 1/4 inch ball valves. I installed 1/8 inch ball valves on the NPT threads in my upstairs laundry room radiators. I purchased those 1/8 inch ball valves from my local Fairview Fittings distributor. If you look under "Hoses and Fittings" in your local yellow pages phone directory, at least one of the companies listed there will stock and sell you a 1/4 inch ball valve. Every plumbing wholesaler in our city or town will sell you a 1/4 inch ball valve if you pay cash too.