Dan, I to am thinking about building a Sportster twin. I ordered conversion plans from RCM, (plan # 930). This is for converting the 40 size Sportster to a twin. I plan to have these plans enlarged to yield an 80 inch wingspan and power it with 2 Magnum 90 two strokes...............Jerry
They made a conversion for the Dirty birdy. Would be about the same for the Sportster. They used the nacelle and cowel off of the bridi commander. I have the cowels, but sold the nacelles if you need some. The cowels are ABS. Would be a good flying plane.