It is fairly easy to copy the tank. You have the bike and the motor is mounted and if it is mounted the same place as the original you just have to look for location points. Get some poster board from the local dollar store first so you can tape up a mock gas tank to see how it looks and fits.
The top and front curves are dictated by the bike frame. Tape a piece of the poster board to something stiff like a piece of card board and hold it up to the frame and mark out the curve of the top and down tubes.
Hold a straight edge of some sort vertically against your computer screen and look to see where the end of the top of the tank ends. His tank ends at the middle of the air cleaner.
The bottom of the front of the tank ends in line with the top of the air cleaner. The curve of the bottom of the tank is your choice. I usually lay the cardboard that has the tank lines traced out on it on an a piece of plywood and put a nail where the top of the tank ends and the bottom of the front of the tank ends. Then I use an old metal yard stick I keep for the purpose or something that bends easily and put it up against the nails and bend it in until I have a curve line I like and then trace the line onto the cardboard.
Get the diameter of the gas cap your going to use. It looks like the builder put it close to the frame and added the same amount to the other side. Just add the width of the frame to the tank width measurement.
Tank narrows at the rear of it. I'd try 1-1/2" less than the front.
Hope this help out. I've used this to make the Indian Camel Back gas tanks I've made. Feel free to ask me any questions you have.
Steve.