Has anyone run a K&B 61, stock number 99-6151 in control line stunt?

Gary,
I don't know about K&B Stock numbers but I assume you're talking about the old side exhaust baffle piston K&B 61.  Internally it's nearly identical to the ST60 and a very good running stunt engine.  I've had a couple of them over the years.  They are about 1 ounc heavier than the ST60 but other than that they are almost a perfect substitute.
I'm pretty sure Tom Dixon offered some for sale with slight mods in the 1980-1990 time frame.
They were and may still be offered by the folks that have the tooling for most of the K&B engines now...and I could tell you who they are if I wasn't having and old guy brain fade...I'm sure someone will come up with the name.
At any rate if you have one and want to use it, you won't be sorry.  Good running powerful engine.
12 or 13X5 APC prop works very well.

Randy Cuberly

The K&B 61 is the same engine as the Veco 61 as far as I am aware. I have one of the HB manufactured "European series" Veco 61s and it is an excellent old style stunt engine. I wouldn't mind another one. As Randy says they run like an ST60, but are a touch heavier. The one that I have doesn't vibrate like an ST60, so what you lose on engine weight, you can make up on build weight by not having to beef up the nose and bearer area.
  I missed a couple of Veco 45s from Byron Barker's estate. I would not mind trying one for stunt if I can find a good low time one.

Regards,

Andrew.

 They are an ounce heavier & run about the same ,,power & rpm as the Tiger .60 & Merco .61.
  John

The K&B Twister is an ABC version of the old K&B ring engine. It isn't the same engine by a long way. I believe the question referred to the older K&B.

Regards,

Andrew.