1971 Kawasaki G3SS engine swap

I am restoring a 1971 G3SS and I'm considering swapping the engine to a bigger one if possible without any modifications to the frame. Does anyone know if there's a direct bolt-in engine out there that will fit? I've heard the kawasaki ke100 may fit. Does anyone know if the ke100 or a larger engine will mount? I'm going to start a complete build thread and post progress pics.