An R1100 With Ural is a perfect and easy combination. We've build one for a friend of mine. The crankcase has two massive mounting holes at the front and the rear.
The upper front mount is a simple thingy on the frame bolt and the swing arm. If you make a solid strut on your sidecar frame you even don't need a rear strut.
You don't need a swing arm, just replace The front ball joint on a adapter plate.
On the rear wheel you can fit a narrow 17" car tire to manage the rubber consumption.
When finished you end up with something like this