I think there would be sufficient market for some of these to merit the project; the port covers, battery-doors for the 41 series, the 75 battery door, also perhaps molded plastic packs for batteries, or of module boxes-- the possibilities are there.
If you are doing the toolwork yourself, rather than simply creating a form directly from an original part, you will primarily concern yourself with precise fit and compatibility; after that, however, it is not only conceivable, but desirable, to IMPROVE, where you can, the design-- and in so doing, differentiate your design from one simply copied. I know that may be hard to imagine right away; do some thinking on the pieces themselves, study them and how they act, and perhaps you'll come up with something.
The improvement/differentiation could do several things:
A. Properly identify your part as a repro-- protecting the market for all-original equipment. As most here are familiar with the demands/wishes of collectors, you should see that this move should not harm your market overall.
B. Protect you from the legal ramifications of "lifting" or copying someone else's design/tooling.
C. Rib or reinforce those areas that may have proven fragile in the field over the lifetime of the part.
D. Provide smooth "label areas" or other feature not included originally in the design. Or alternately, a molded-in legend providing useful information on the calculator's operation... one can imagine the self-test and reset and other subtleties of a calc being documented on a battery door, for instance.
E. Create a brand-image and repute of your own-- so that the hard work you do in this area will translate to a recognition amongst collectors that your parts are valuable in their own right... a small but recognizable logo or "cartouche" so that the source of such a part is known and respected... that would be better in my mind than a "source unknown" knock-off part.
The project has merit, surely. Best of success to you-- looking forward to seeing examples of this being done right. If they are, I'll be a customer, too.