Mounting a motorcycle fuel tank isn't difficult but you really need access to a welder and be able to fabricate mounts. I use motorcycle fuel tanks on all of my bikes and they are all different and required different mounting techniques. I've used Susuki, Yamaha and tanks designed for Harleys.
What fuel tank are you considering using? How did it mount to the original frame? I always try to duplicate the original mounts on my bike frames to match the fuel tank with exceptions. I've modified a couple of tanks to match my bike frames.
Good luck and be creative. Just be sure the mounts are secure and be careful welding to/on a tank.
Tom