To save cash, I'd go to a salvage yard for a tank. Just figure how you want to bolt it up, measure your main tube, hieght , length and were you want your seat and tank to meet.you could even do a cardboard cut out of your height and length to match up to tanks till you find one you like.. I like the old BMW,Norton.and the Hodaka tanks.. If you want shiny , maybe a old Hodaka ( 60/ 70's) thank would be the way the go . The tank has a endent for the hodaka emblem, but you could always made a round R.E. Decal to fit in it. To bolt a tank up you have a few options the stock Nfield circle up front are thru holes on the frame, straight bolt thru the frame is a option, in the rear above carb are mounts , I made L brackets there for my tank and they are pretty well hidden still..you your imagiation for your design.. Another tank for shiny ness is a old husqvarna tank, just woould have to stripe the paint off, their all chrome under paint, a 1978 tank from a 390OR would be a good choice if it fits the tube length, their a bit longer and narrow. Remember you can always put rubber down on your tube to change the angle of the tank too.. Both the husqvarna and hodaka tanks have long tanks up front so you could do thru from bolt on the stock mounts.
I have the cafe kit on mine, and the tank gets warm( AZ Sun), and always have to polish it.. For 1300 I'm not doing a do over..I already replaced the solo seat and made the Airtech seat, set up for dual riders. I Had to add to the seat pan and do a bunch of glass work but it came out good..it's perfect set up for my wife and I .. Keeps her close and she can't slide off the back still.. LoL
Sorry for rambling.. Hope you find the tank you desire, there's plenty out there pretty cheap..
Jason
P.S. Here is a picture of my one L bracket at the back of the tank, the rubber him met I got at autozone and knocked the brass sleeve out of it to give the tank some padding..