I think if I wanted either a side car rig or a trike, I'd base it on a Gold Wing. I know it would cost a lot more, but there are various companies with considerable experience working with that platform. The runner-up would be Harley - but I'd prefer the stronger, smoother, water cooled Wing engine.
We had a guy in Connecticut get seriously injured a few years ago when the front mount of his sidecar rig on a Moto Guzzi let go and dragged him into a guard rail at considerable speed. I would want the strongest base platform and mounting system available.
My 650 has seen some reduction in both mileage and top speed just from adding the big XXL Clearview wind screen and the Givi trunk, which both create wind drag. I think the main factor is the wind screen. Adding a sidecar would definitely impact performance and mileage.
Handling would be affected significantly also by either a sidecar rig or a trike conversion. I'm not sure I would like that. But for some folks, especially with leg, knee, or hip problems, the added stability is worth slowing down a bit for. I understand that, because I'm willing to trade a few mph of top end and several mpg for the greatly enhanced comfort of the big windscreen.
That said, I heard of a guy back in the sixties that did some significant touring on a 175cc 2-stroke Lambretta scooter with a side car rig AND pulling a small trailer. You know he wasn't moving too fast, but he did get where he set out to go.
