don't sell it!
You have a very nice rig that is nearly finished. The R90/6 machine is possibly the most long-life airhead, has very good power for a light to medium sidecar, and this example has the large touring tank which is essential for a tug. You can make the rig much easier to ride by replacing the stock triple clamps with a set which reduces the rake of the forks. Perry Bushong of Perrys Motorcycles and Sidecars in Fort Worth, Tx. makes them and sells them. Perry was the BMW Motorcycle Franchisee for 30 years and has built many BMW rigs equipped with these and they transform the handling, its like power steering. Add a plexiglass windscreen and you can tour all day on your rig.
In our market, (Dallas Fort Worth area), a rig like yours which has Perrys' front end set up would bring somewhere around six grand. If you wanted to have one just like it built, it would cost you at least ten grand. If you could find a pristine R60/6 that is. They are getting hard to find in such good condition.