I don't agree with having to convert to 12 volt with the 1600. It would be easier to redo the 1200 to be sure as it would only be a top half rebuild. A set of pistons, replace the exhaust valves, replace guides if need, 3 angle the seats, and reassemble. The 1600 on the other hand would, in most cases, require a complete teardown because it ate a valve head. Possible bent rod, crank journal damage, plus the cost of a new head and new pistons and cylinders. If you have the tools and skills to rebuild the 1600, I personally would go that route. You can use the 6v ignition system, coil and distributor. Generator and housing can be used. 6v flywheel will need to be used along with other pieces of sheetmetal and the 1600 muffler. It's just a matter of your personal preference, time, and cost. The 1200 being cheaper.