Just any small flat file should work. I bought a set at gun show specifically for gunsmithing. Check Amazon or even harbor freight if you have any close to you. They are around the diameter of a writing pen, maybe a tad bigger. Heres an amazon link of the same ones i have. Im new here and its making me wait 10 mins to post this apparently:
I'm about to start building one of these once my Form 1 is approved and I'm wondering about filing the hex pattern in the endcap. Should I just use a needle file set like this one?
I have a Jinhao pen as well
and it used the same screw in refill as what yours likely does (5th pic):
https://www.amazon.com/Gullor-Culture-Jinhao-Rollerball-Pattern/dp/B00CHXK7WY?th=1
Great pen, quite remarkable for the $20 I spent on it, but the refill is not good. This is a common trait on high end pens as well, including Montblanc's.
I used needle files to widen the inside of the nose piece to accept gel refills like Pentel Energel, Uni-ball Signo. (like this: https://www.amazon.com/Hardened-Strength-Barrette-Crossing-Equaling/dp/B07PPYWSCY)
I'd probably recommend an assortment especially for light work. Files don't wear out quickly, and you never know when you'll need a different shape. I use a cheap set similar to this.
https://www.amazon.com/Hardened-Strength-Barrette-Crossing-Equaling/dp/B07PPYWSCY/