My 100 hp Rotax 912s in my flying machine weighed in at 67 kg. I doubt if the 90 hp K100 engine can get down to 100 kg, and it was not designed to run all day at max revs like the Rotax is.

A bit heavy for what they put out.  

They may not be specifically designed to run all day at high RPMs but they certainly will.  Without a 6th gear it's the only way to make good speed. 

The interstate speed limit in Montana is 80 MPH which means you can do 89 (143 kph) all day without getting a ticket. That's a hair above 6,000 RPM on a K75.

Complete K1100RS engine, sans gear reduction for aircraft, radiator & coolant, ignition module & computer, but with injection components, coils & engine loom, on my precision German-made bathroom scale. The ultralight engine shown doesn't appear to be using an alternator or traditional starter, the propeller likely being the method of starting. (158.1 lbs)

Don't know if it's an urban legend, but I've heard that K engines were designed to run 24 hours at red line. 

I've run my K75RT for 14 hour days at 6000+ rpm stopping only for gas.  Have done 100mph+ for well over an hour at a time in Colorado/Kansas and up in Manitoba/Saskatchewan.  Even with 94,000 miles on the clock I have no hesitation to open up my K75 in desolate places far from home.  Heck, isn't that what they were built for?

Well they did start out making aero engines:D

Somewhere in the Walker and Dobson book is the statement that the K100 engine was designed and built to take twice the bhp than it actually puts out.

Complete K1100RS engine, sans gear reduction for aircraft, radiator & coolant, ignition module & computer, but with injection components, coils & engine loom, on my precision German-made bathroom scale. The ultralight engine shown doesn't appear to be using an alternator or traditional starter, the propeller likely being the method of starting. (158.1 lbs)

The Rotax 912S does have a starter, and a reduction gearbox and a alternator. And 67 kgs is with coolant and oil. Starting with a propeller is something that was done in the 1950s.

If the K engine could shed around 40 kgs then it would probably be a contender, but as it is people go for the 912s for a reason.
 My whole aircraft including engine weighed less than a K1100LT.

Had a mate with a Tiger Moth and swinging the prop was the only way to go for starting...had low compression on one cylinder as you turned the prop through it's rotation prior to starting and it would be hard to pull through 3 cylinders and then would just flop through the 4th; so we did an easy valve grind on an upside down donk..would have cost him heaps to fly an engineer up to do it, but it was an easy fix with a slow speed drill and some valve grinding paste. K donk would have been nicer if you have ever been in a Tiger Moth.

Ten posts in and no "Flying Brick" comments? 

Well, you just made one?