I'm not familiar with that model, but I've noticed on the Targa LX vs the Sprint that the two main differenaces seems to be:
The top tank model has mounting tabs welded on that the tank pivots on.
The step through model uses the front part of the chassis as a gas tank, and therefore has a gas cap on the chassis.
The problem is, that if you want to weld the mounting tabs on... you'd be welding RIGHT on the side of what they were using as a gas tank. Needless to say, if your going to do that you better fill the tank with a noncombustable liquid first. (prefferably one that won't hurt anything else either.
I'm not sure if the seat is different or not. But you migth have to add the little gas tank mounting bracket thing.