A double sided pin vise should be able to hold any drill bit you have... My smallest drill bit is a #78 (.016) drill, and the tighter end of my pin vise has no trouble hanging on to it...
But I am a bit confused... Are you trying to repair the wing tip fins or the ones from the upper air intakes (the things above the wings and on either side of the cockpit)?
If you are trying to repair the wing tip fins, there is a simpler way... You are almost correct when you say the Unseen Samurai and Sparrow Hawk share the same wing tips. Actually it is Unseen Samurai, Sparrow Hawk AND Corsair. While not perfectly identical, I have previously purchased the newer Corsair wing tips from IWM. I figure if you know how to get Seydlitz fins, you can get these. The difference between the older and newer fins are slight, and would be no more noticeable than homemade fins.
If you are trying to repair the upper air intake fins, I would just trench a slot and glue in a cut down Seydlitz fin.
Either way, I'm not sure you would even need a pin vise to do either.