Tim, I have 2 new stock HB .61s that were given to me. They were from the Army target drone program about 20+ years ago at Auburn University. The only oddity about the engines is they are not bushed like most of the drone engines in that program.
Barry -
The two HB .61s I have are slightly different. The one that came to be unrun is twin ball raced. The other, though not confirmed, I suspect is ball raced in the rear and a bushing up front. It is said the drone HBs had a rear BB and a bushing in the front that can be removed and a correct sized BB installed. The unrun HB came with an HB carb, the other had no carburetor. I swapped the HB carb and muffler to the used one I suspect as being a drone engine and it throttled poorly. I adjusted the needles quite a bit and never got it to throttle in the midrange well. I'm unsure what's wrong with it but may be the reason I acquired it. It has good compression and turns over pretty smooth.
The pair of Sportster .65s I have are from the most recent drone planes from around '91. One is unrun and the other has about 4-5 gallons through it and runs really sweet with a good carb in it. The drone carbs are junk.