I simply make a sleeve out of drill rod to fit over the air hammer chisel shank and turn the end of the chisel down to fit whatever hole it needs to pilot in. Sometimes a couple of different pilots will work on the same shank- - - -maybe 5/16" and 3/8", for instance. Stovebolt and Chevy V8 valve stems are 11/32" (.343") so I always kept dedicated tooling around for that size. Once the driver sleeve is made on the lathe, I harden it- - - -in either air or oil, depending on the base material. I always used whatever air hammer the Snap-On tool truck had onboard, with a good warranty, but I was doing enough business volume to justify buying professional-grade tools.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!