An easy solution for this, especially with the budgets involved, would be an in house CNC machine, rather than relying on outside sources like Grainger.
Bombadier may have made the current fleet, but there's no reason why Disney shouldn't have designs down to the metric level programmed into a CAD somewhere by now and be able to reproduce parts easily, especially high use parts like door units, etc.
While I tend to agree with the notion that a 20+ year old monorail fleet is probably ready for retirement, it doesn't excuse poor maintenance. And, for a unit that was custom to begin with, I fault one of two things...
1) There should never be an expectation of "regular parts" being available, and therefore a supply for on demand irregular parts should be available
or
2) The system was poorly designed (not the exterior fiberglass) in the first place to use uncommon parts, if the expectation is that the parts should be readily available.
And that's just parts...the other aspect is cosmetics. It doesn't take 24 hours to re-carpet, or to detail clean a car. Especially if you toss a well organized crew at it. Hence why a regular maintenance cycle, as opposed to running them into the ground before refurb, is a much better solution. Albeit more expensive, in terms of man hours, but a better solution.
Just like keeping a duplicate computer loaded and ready for swap, while more expensive in terms of hardware and software, is better than downtime.