Thanks for the help on this. I have been on a fascinating trip down the byways of the internet, reading about Maynard Hill Sampey radios and Fox engines.

It looks like he used a standard short shaft Fox 59 for at least one of the altitude records. The Fox story makes no mention of a special engine for Maynard Hills record breaking attempts.

It does seem amazing that you can set the needle at launch and still have the engine running with enough power to continue climbing at 25,000 feet. I did find a referance to Hill's use of high nitro fuel so I guess that provided some or most of the oxygen necessary for combustion.