Not a P.C thing but frames with out top bars are also called "step-threw" frames.
I built a MB for an older guy and he was all "ell NO!" when I showed him the bike. As it was a girls. I said that it was not a girls bike. Rather it was a step threw (sp?) and all was good.
Dunno if that would help in searching.
Ya can buy top bars to mount a tank to. But a can tank would be far easier. They mount to the seat tube and sit behind the seat. Easy-peasy and cheap. Can make one from a can but are sold, as well. And they look really cool.
Without the top tube, ya have much less restriction. Big plus, there.
EDIT: or fab a top tube to hold a tank.
Any kit I know of, the gas tank just has to be higher than the engine as they are gravity fed.