Rad shop or acetone or just get this and follow the instructions. Probably a cool idea to do the whole Por 15 thing while you're at it.
I'm also running the 12.5 gallon tank in my speedster. Just like @edsnova suggested, I sloshed the new tank with acetone (and some bolts) twice and then did the POR 15 treatment. Absolutely no problems in nearly 4000 miles. Also, like @WOLFGANG, there's no room for a spare. I just store a small electric air pump and a tire repair kit next to the top of the gas tank for emergencies. I'm getting about 300 miles to a tank full, but that is truly on fumes.
Incidentally, I didn't feed like the CIP gas cap gave me a tight seal so I found a replacement. The Stant 10640 radiator cap fits the 12.5 gallon gas tank and leaves room to spare underneath the hood.