Yes, the 3M battery can be rebuilt, but the only caveat is that the rebuilt battery won't be certified as intrinsically safe ... which would only be an issue if you were working in an industry subject to MSHA and OSHA regulations such as a coal mine. Somebody posted a thread here several years ago. They found a business that rebuilt batteries and they replaced the four "D" size Ni-Cad cells with ones that were basically equivalent in mAH rating. I think that you would be able to open the battery pack with an X-Acto knife to break the seal between the top and bottom. The Ni-Cad cells that you need are the type with solder tabs. You might be able to talk the people at Batteries + into doing the job for you if you can convince them that it doesn't need to be intrinsically safe.
I don't think that the Li-Ion batteries are the best option for the Airstream mainly because of voltage mismatch. The Ni-Cad battery provides approximately 6.3 volts initially at full charge and 4.8 volts near the end of charge. A Li-Ion fully charged cell voltage is approximately 4.2 volts and 3.4 volts at cut-off. There are some Li-Ion power packs with a built-in voltage regulator to provide 5 volts output which I believe is what
@alan Zenreich used. My only concern about that type of power supply is that the output voltage is at the low end of the normal operating voltage range for the 3M AS-400 Airstream. Also, since I now live with neck pain I don't care to add weight by mounting the battery on top of the helmet. I don't know how difficult it is to locate a suitable power pack. I think that the particular model Alan used is no longer available.