You will have to measure the metal component diameters at the points closest to where they meet the pen blank ends.
You could do without bushings by using calipers to monitor the blank diameter as you turn it down.
To find compatible bushings, I would suggest searching for kits with names like "Round Top Streamline".
I also note that the refill carries the Dayacom name ... you could search the website ...
www.dayacom.com ...
... but that is not a particularly easy chore.
A search that I just did for "24 K Gold Mount Blanc Style Pen Kit" brought up this result, which looks very similar to your kit : ...
"Twist" mechanism. This old world style pen is the one to choose when enduring quality is your standard of performance. The 24kt gold offers a bright pure gold plating that will give your pen a rich and luxurious look. 24kt gold uses a "premium rack plating" process and is covered with an epoxy...
www.langevinforest.com
It would seem that ...
www.langevinforest.com ... is a Canadian company in Montreal.
Another search reveals that ...
www.pennstateind.com ... used to have a pen kit called a "Designer" which used bushings PKMONTBU that are still listed on their website if you search for the latter name. . Those bushings might suit if you can get them. . A phone call to PSI would help.
Another company to try is ...
www.woodturningz.com ... and look for PKMONTSG or EUROBU .... VERY similar to what you show but I don't know if they still carry those products.