after digging around a long time, easiest option I found was as Master noted. get a piece of 1" keyed shaft, 2 #50 keyed sprockets and some 1" pillow block bearings.
www.*************.com about the cheapest route. very simple to setup, hardest part is milling down the sprocket to match the engine sprocket to 520. if tension/adjust worries are an issue, a pair of tubes welded in front of the bearing saddles with adjust bolts going thru them to the front of the saddle is an effective and simple adjustment locking device.