Just reporting back on the refill for the Calcu-Pen - Great News !! it works.
If you take a modern ballpoint refill for a multi-function or mini pen the diameter is the same it's just too long.
Measure the new refill against the old refill, mark and snip. I used a glue stick to seal the end, I guess you can use putty or something else.
The biggest challange is keeping the roundness of the refill when cutting. I had to use pliers to keep it back into shape, however next time maybe a plumners copper pipe cutter (small one) or a dremel tool would do less damage to the shape
Just reinsert into the pen your good to go with your new refill and it looks good ! You can even pick a color or use a highlighter refill.