Unless the threads have little or no vestige of usability left, the best answer is to use a chaser, also called a rethreading die.
If the headstock were sent here the first thing we would attempt is to chase the threads. We urge you to attempt it, or get a machinist to help. This is the first thing we do when this problem is encountered.
The rotor/spindle can be removed, but it is not a simple task to remove or replace. Our service center is in St Petersburg, Florida. We would require the headstock to be shipped to us. Shipping a DVR headstock requires special packaging, as it needs to be double packed as if it were 100lbs of eggs. Pack and ship places charge 75.00-90.00 to meet the stringent requirement. There would also be parts and labor charges plus return freight.
Here are four places in the US to acquire the correct size die, 1-1/4" x 8TPI UNC. You will also want to use a die wrench to hold and operate it. If the problem is with a Nova Comet II, the thread size is 1" x 8TPI UNC.
Gyros (92-98008) 1-1/4 - 8 Carbon Steel Hex Rethreading Die
Gyros 92-40108 High Speed Steel Die 1-8
Here are some DIY videos:
How to use a tap and die - FIXING STRIPPED THREADS
MACHINE SHOP TIPS #1 Taps tubalcain
MACHINE SHOP TIPS #4 Dies/Threading
MACHINE SHOP TIPS #5 Dies/Threading