How did you diagnose the threads being off center?
Ross, If I’m understanding correctly, it’s hard to imagine spindle threads being machined off center. 1mm off-center would be an almost unbelievably horrible error, causing a shift of 2mm from one side to another. The outside of the chuck body itself or a cylinder mounted in the chuck should immediately show the offset. If more than one is that far off, Rikon must have machined a batch that way.
I’d check with a dial indicator. Mount something on the spindle and check/measure the runout with the indicator mounted in a magnetic base. If you don’t have one a friend might, but some are inexpensive (I see some with base for $40 on Amazon) - I keep some for my little machine shop and one near the wood lathe - really handy, especially when remounting a piece in a chuck. (If a friend has a dial indicator, see if they would come to your shop to do the check - I’d do it if I lived nearby.) BTW, get a “dial indicator”, not a “dial test indicator”, a different and more precise tool but not needed for this type of test.
I’d mount something like a Glaser screw chuck, a spindle extender or adapter, a threaded mandrel (bottle stopper, etc), a chuck, or a piece of metal rod mounted in a chuck. Turn by hand. The dial indicator would immediately show and measure any error on the body or the rod. If in doubt, take the indicator and adaptor or chuck to another lathe and compare. (Note that it’s not uncommon for chuck bodies to be a few thousands off center or out of round but shouldn’t be anything more.
Could also mount a short block in the chuck, turn it round, then reverse and hold in the same chuck and turn the other end round.
Jacob, if the replacement spindle is running true and just missing the spindle lock keyway, even not seeing the spindle or the spindle lock configuration, unless the entire spindle is hardened tool steel I suspect you can make a keyway - a friend with a mill, a local machine shop, or even use a drill press or hand tools. A thread burr might be removed with a file or a bit in a Dremel.
JKJ