Does anyone know if a Sputnik sidecar would be an easy hook-up to an S40?

Savage , S-40 will carry a Sputnik nicely. Expect gas milage to drop to about 45 MPG. Top speed will be in the seventies but you should be able to cruise all day at 60 without harm to the engine.
I would suggest a change to chain and sprocket at 16 X 50 ratio.
These are the ratios I used to travel twice from Coastal Washington the the center of Montana, and twice up into Canada and twice down into Oregon. All trips carrying about 150 lb load in the sidecar.
Be sure the toe in and lean out are correctly adjusted so as to have no pull to either side on flat level at 55 MPH.

Phelonius