With no sign of a way to get a current-model inner cap for a Pelikan M200 (see my other thread), and no desire to have my red pen out of service for two weeks or more because Chartpak insists on having the whole pen, I finally decided to take matters into my own hands with it.
Obviously, given that I don't have an ultrasonic bonder (although I used one briefly in plastics shop at the University), there was no way I was going to get the inner cap back together in a way that it wouldn't break immediately under the stress of reassembling the cap.
At least, not unless I reinforced the repair in some way.
I looked around my workbench, and found a piece of brass tubing, just the right size to fit tightly inside the inner cap. So I carefully turned the diameter down a bit in my drill press (= poor man's lathe), so that it would fit into the broken-off tip, and so that it would be sharp enough to plow its way into the tip, just a bit.
Then I used CA to glue it to the tip, allowing it to set for several minutes. Once it was firmly attached to the tip, I then slid it into the broken end of the inner cap, lining up the break as best I could, and poured in another drop or two of CA, making sure it was able to soak in between the tubing and the inner surface of the cap.
Finally, I allowed it to set for several hours. Then I tried screwing the nut on. It was very reluctant, but it would go on far enough with a bit of elbow grease, and come back off.
Then, I let it set for several more hours (to make sure there wasn't any uncured CA left, that might glue the inner cap to the cap body, or (worse) the cap body to the barrel, and finally inserted it in the cap body, added the clip, and torqued down the nut.
So far, it's holding. Only time will tell.