84 Kawasaki

The Japanese were not known for making "universal fit" on things like frames. Even if you found a frame that fits the engine, you will run into other problems in getting the front end to swap over and then fitting the rear wheel and getting the rear wheel sprocket to line up with the engine sprocket. Why not consider hopping up the KZ440 engine instead?
You can likely enlarge the engine displacement, install hotter cams and add a header for more HP.

If you’re going to build a cafe racer, it’s easier (and usually cheaper) to start with a more or less complete bike.

Buying everything separately will be time consuming and expensive. Not that starting with a donor bike will necessarily be cheap. It’s the little things that add up cost wise!

There isn't a bigger direct fit engine. Already said, but just get a complete motorcycle.