If it is a tapered pipe thread it will be about 1 1/2" pipe. Pipe is measured by the inside diameter. You could solder the fitting on too.
2 1/16" is a non-standard size. 2" plumbing/electrical pipe is actually 2 3/8" OD, 1 1/2" pipe is 1 7/8", there is no 'standard' size in between. If it's a NPT (national pipe thread), you could use 2 1/16 OD pipe (if you can find it, which I doubt) and find a friendly plumber with a pipe threading machine with an adjustable die to cut the right thread. If it's not a NPT and/or you can't find the right size pipe, then you'll need to have it machined after figuring out the thread pitch. If you can get the threaded bung out of the tank, soldering/brazing would be the easiest fix...
Is that a cork gasket under a flange? If you can remove the original threaded hole. Maybe get a bolt on one: http://www.ebay.com/itm/New-Bolt-on...ash=item35d03470d7:g:IZMAAMXQKQhRGqSX&vxp=mtr
...think you might be looking for this: Made for all our Model "T" tanks. Screws into the cap bung and allows the attachment of 2" hose for a remote fuel fill. Works great with our flip top and deck fills. Machined aluminum. Made in USA. http://www.thehotrodcompany.com/shopnow/show_item.asp?product_id=T505
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