Case machining alone is cost of a complete FE golden gun, just machining, then, sanding down all polish, assembly, engraving, gold plating, even a 1 micron plating on case will be expensive, as it's working, lots of area to cover.
So it's not possible.
And other problem if case upgrade, other parts will look cheap.
I can make a matching bullet, this can be done for affordable cost.
I have a FE 2010 here, didn't even try to put my bullet in, will take a look.
When I designed the pen, the thread size design has increased several times, from 9 to now 12mm, and my previous size is matching the FE gun.
I upgraded it so I can use my pen as barrel and put the bullet in.
But I wanted to use a 11mm thread again, to make collectible pens, usable as FE replacement, including working removable nib.
I will make a pen video soon, including reference searches, design change and last pen version.
I would make a run one day, if I get enough people to make it for decent cost (1500€ would be great, my actual one will be more than 2500€, bought by friends, they pay the machining and printing directly) I changed design for a simplier one so I cold reduce the cost, but to get it efficient, would require at least 20 people.