Talk about 'rare spares'. This oil-cooled Windhoff 4-cylinder ohc engine was found in Sturgis, South Dakota, propping open a barn door, which several hundred thousand Harley-crazed bikers had stumbled over in their search for a Knucklehead supposedly inside the barn itself. Just kidding. 
The Windhoff is an impressive machine from the late 1920's - I first came to know of this marque when a complete example was found in a basement in Chicago (?) in the 1980's. The Windhoff has risen from total obscurity to absolute cult collector status in the intervening decades, and just last Spring, a restored 1928 746cc Windhoff sold for $177,000 by Bonhams, at the Stafford show, making #12 on my 'Top 20' auction list.
The creations of Hans Windhoff began in Berlin with radiator production for cars, trucks, and aircraft. In 1924, they entered the burgeoning motorcycle market with a ladepumpe water-cooled two-stroke machine in 125cc - an excellent although expensive creation, with the engine built under license from a design by Hugo Ruppe, whose ladepumpe (extra charging piston to compress the fuel/air mix) design was used most successfully by DKW in their GP bikes. The factory had much racing success with these smaller two-strokes, although enlarged racers of 493cc and 517cc were less reliable.
By 1926, a totally new machine was offered; the 746cc overhead camshaft, oil-cooled 4. Only Granville Bradshaw (creator of the ABC) had successfully used an 'oil boiler' engine in a motorcycle, and the new Windhoff was a technical tour de force. The engine, designed by Ing. Dauben (later to join Mercedes) had no external 'plumbing', using internal oilways to keep everything cool, and all castings and pistons were in aluminum.
The chassis construction predated the Vincent concept of engine-as-stressed-frame-member by several decades, as the motorcycle is built around this large lump of an engine, with a cast headstock bolted up front, and the rear frame tubes emerging straight out the back of the gearbox castings.
Front forks use leaf springs, with no rear suspension, and shaft final drive was employed. All-up weight was 440lbs, a bit heavy for the day, but certainly not by today's standards, and the 63x60mm short-stroke engine produced 22hp at 4,000rpm. Apparently it was a very comfortable and durable touring machine, with excellent handling, undoubtedly due to the rigidity of the central 'frame' member!
The price when new was 1,750DM, a bit more than the 1,600DM of a BMW R63. A bit expensive, a bit unconventional, and a bit slow on sales, nonetheless the machine ranks as a landmark of vision and development, and is understandably very sought after these days.
This engine will no doubt form the basis of a new machine - such things are not beyond the wit of man - and there are a number of complete machines extant to copy; I've seen three in Germany. The photos of the complete bike were taken in the excellent Deutches Zwierad Museum in Neckarsulm; worth a visit if you have a spare engine! The free-standing engine is under restoration in Bavaria; all photos were taken last October, 2008.