I have an old typewriter made in about 1912. The robber platen was
rock hard so I checked into various ways to rejuvenate rubber. There
are proprietary formulas as well as several do-it-yourself methods
on-line. As long as the rubber can be softened before it crumbles or
cracks this can work.

Another possibility is to print the parts with elastomeric filament.
There is a company that does elastomeric 3D printing provided for
information mainly. There are also liquid resin 3D printers that could
provide more choices of elastomeric material.

Elastomeric filaments are also available for the higher end consumer
printers. You would need to check compatibility with your printer since
different filaments require different extruders.

If I were doing this for my own use, I would start by trying to find a
friend with a suitable printer so the properties of the material can be
tested before investing in modeling the parts either by hand drafting
of by 3D scanning.

I don't know what material the cup may have been made of over the
years but the leather worked fine and suitable leather can still be
found. I would reduce the number of parts needing to be printed to
the essential in case they are expensive to have made. The nozzle that
fits the tracker bar obviously needs to be flexible and air tight.

I sort of remember that there is a rubber check valve that can probably
be replaced with a similar check valve taken from old medical equipment
and glued in place of the failed one. I have some of these valves
salvaged from medical equipment (suction devices best bought new old
stock not used) and they are usually made of silicone and will last
many years.

Spencer Chase