I'd MIG it. still fairly pretty and they strength isnt an issue since yamaha alloys their frames with chinese newspapers.
but if you were to graft one frame half to another id think all youd need would be a welder, varying saws, and possibly a bender for reinforcing. but you could skip on te bender if you used .125" plate, instead of round tube.
i dont see it being that hard, but if you were to start from scratch it would be a different story.