Should only be easier with todays technology.
Blueprints will contain all the measurements needed.
, then it will only be easier with today's CNC technology to produce an accurate canister.
Presumably the blueprints or the files show what the finished Cartridge parts look like. To make a mould you have to know how the actuall plastic flows, (so that their are not any Void areas where the material is too thn or missing. You have to know where it will shrink as it cools, so you can make the cavety just enogh bigger than the finished size so the plastic will shrink to the exact size you need.
These parts have some critical dimensions, but have to be cheep as they are only used once, so no machining of the finished parts is allowed. they have to pop out of the mould and be ready to use with just a tumble in Walnut shells to get the Mould flash off.
Now on the plus side their are some very skilled manufacturing staff in places like Italy, But yes a 100K dollar investment might be needed to make a small (3-6 cavity) mould for the two halves of the cartridge, and some more to make the fancy Mould (four movable parts) for the take up spool.
I am not sure if the requirements for 126 backing paper is all that much different than for regualr roll film. So if they are running down a suplier so they can make 120, that suplier can also probably make 126 paper with only a few thousand dollars of additional tooling expense. Only thing I don’t remember is if 126 Paper had the perforations to match the film, if so the precision required goes up a couple of orders of magnitude.
The chances of seeing 126 would have been MUCH greater if the Original tools had turned up, although they probably were 12-25 cavity items where the moulding plant would probably take more time setting them up than in running to make the new normal production run of a couple of thousand units. (it is possible that the cartridges were made by the Spanish Folks we know as AP photoplast, but they seem to have become invisible on the market.)
In loading Super 8 Film, some of the small suppliers still buy their super 8 cartridges from Kodak. Those have about 6 parts, plus a metal piece. Since Kodak no longer makes 126, they proably no longer can provide the empty cartridges.