I can't tell from that pic, but it kinda looks like that bike has a perimeter or backbone frame. You want a frame under the engine (cradle frame), in my opinion.
Another bike you may wish to consider is an old Honda XL. I mean old XL (70's-80's). The world was lousy with them, so you may have good luck sourcing one on the cheap. Good reliable power plants. Kinda heavy, but stripped down, it might just work well. My first bike (that I owned) was a '74 XL100. Beat that thing like a red-headed stepchild. Nearly bombproof. Perhaps more of a scrambler style, but it might work. For example: