One word, plastics.

[Belial6]

It seems kind of silly to ask if anyone has thought of making new parts, but telling them not to mention the company that does.  Pretty self defeating.

Ignoring that, the reason that you can get replacement parts for a retro car and not for a retro computer is cost.  The setup is the biggest cost.  If there were as many Amiga owners needing replacement door panels as VW Beetle owners needing replacement door panels, and those Amiga owners were willing to spend as much for a door panel as a VW Beetle owner is willing to pay for a door panel, you would see replacement panels panels made available.

[Belial6]

For a part that will not be touched extensively, paint may just be a reasonable option for color matching.  I know that having our plastic the right color is better, but a $3 painted part that you do have is certainly better than a $300 solid color part that you don't have because it is too expensive.

I would think that getting the right texture would be the biggest barrier.

[Belial6]

If your going to do a simple pizza box, and are going to do the whole case, it seems to me that a CNC would be a way better choice than a 3d printer.