FYI that I spoke to Pamela at ISM and they no longer stock the X27-689 and have no plans to have any more manufactured (as she put it). They do still have the previous generation X25-689 which is what I bought. They are $10/each. The differences are minor and total drop-in replacement between them, so its internal differences I won't even notice.So order the X27-689. The motors are $12 each.
Link to X25 vs. X27 diferences
I am going to replace both, even though my tach is the only one showing issues. I'll follow your instructions and report back in a week or two when I have it done.