Biggest carb that will go on a 200 rotary, when fully dremelled out, is a 30mm. Any bigger carb will obviously still run but won't make any difference over a 30mm. That being said an SI26/26 doesn't have a manifold, so equivalent to a 28mm sidedraft at least. Bigger carbs just have better acceleration and as this is of less concern on a sidecar engine, SI26/26 will be plenty I'm sure. As said autolube an added bonus.
One of the quattrinies would be well suited but all the faff with cranks, machining and the ever growing bill, just won't be enough better than the MHR. And as this gsf, probably less at the top and could be slower.
Whatever way you decide, it's going to be interesting.