Post a photo of your current tub spout. You noted a fine thread so I'm betting its an older unit as the 1/2" NPT by fine-thread spout nipples are purely retro, nothing new uses that configuration.
Pick up a 1/2" Copper x male adapter and solder it to about 9" of copper tubing. Unscrew the existing spout nipple and screw in the new assembly after wrapping the threads with a few rounds of Teflon tape. If you can reach the hex on the copper x male adapter then tighten it there, if not, use some channel lock pliers to tighten it from the very end of the 9" pipe and trim off what was mangled with tubing cutters (I noted to make it 9" so that you had 3" to 4" extra tubing to spare for this specific purpose).
Pick up a slip-on tub spout with diverter to install. They slide over the pipe and lock in place by tightening with an Allen wrench, be sure to get it level because you'll notice it when bathing. I'll usually dress the end of the tubing with some sand cloth to remove sharp edges so the O-ring can't get cut when installing the spout. Slip-on spouts are better than the old threaded ones because you'll never wind up with gap or it being too tight.
Select a slip-on spout like this:
https://www.homedepot.ca/en/home/p.d...001031133.html
I interpreted that the OP probably has one like this:
http://www.lyncar.com/plumbing/index...oducts_id=1721