If you're thinking something along the lines of... "didn't he say he was done with the bike and it's ready to be sold" you're totally right. BUT firstly with an old bike, you're never done and secondly the original fuel taps were at least 30 years old and after turning them on and off a few times they decided that leakage was the way forward.
I've built some fuel-tap adapters in the past and remember one set of baseplates that I ruined ages ago, by drilling the hole too large (as in 19mm, where it should have been 16...)
Well then, grab a bit of 20mm ally roundstock, turn it down to 15.90mm and thread it M16x1.
Prepare it for parting off. (My parting tool is an old HSS-blank, which I should really upgrade to some good carbide tool rather sooner than later as I have a few ideas, which involve stainless...)
... and after sound all so professional realize, that the backside better be flat, but with a light chamfer. *doh*
Nevertheless the thing that can be learned from this mishap: a bit of
painter's masking tape and a very gentle hand on the chuck can save
things. Ideally don't f*ck the part up in the first place and you don't have to freestyle your way out. 😉