Re: What needs to be reinvented?
If they don't already have a blueprint for the item (or other instructions on the build process), then I'd rule they have to make an Invention roll, probably at a +5 bonus, maybe more for certain items. This is pretty much regardless of what the actual item is they're trying to make; the Invention roll is how blueprints are created in GURPS, even if the person making them already has a basic understanding of the workings of the finished item or even an example of the item sitting right next to them.
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