ENGINE SWAP! Donna do it laddie. I once tried to stick a KZ750 motor in a 1971 Husqavarna frame, and boy. did I learn a few things...like chain alignment, swingarm position, motor mount insanity...I'm sure since you want to swap a bigger motor into the same marque thinks sorta line up, but you can't imagine what 1/4 inch off can do to motor mount bolt holes, air celaner set ups, gas tank mounting, etc.etc.etc. It can drive you to drink. You also need to be an excellent welder, and have a whole shop at your disposal (grinders, drill presses, vices, bending tools,etc.). Why not just buy a 1000, and stick all the 600 insignias on it?