The only motorcycle-style tank I've heard of fitting on the heat frame is one from a 90's H-D Sportster 883. It's pretty fat but not very long and will fit in the faux tank's space if you mess with the seat mount a bit. Check craigslist, I did for the hell of it and found a guy parting out one that would work, but he still wanted $100 for the tank.
I used a couple pieces of angle iron to make a sort of "shelf" behind the seat and bolted the tank to that. Maybe give that a try.
I used a couple pieces of angle iron to make a sort of "shelf" behind the seat and bolted the tank to that. Maybe give that a try.