The Sioux valve grinder I used to have had an accessory attachment for regrinding lifters, but I don't think I ever used it. If a lifter is worn flat, or definitely if it's concave, it's worn out. Replace it! Lifters are supposed to have a slightly convex shape, higher in the middle than at the edges, so they will spin as the cam rolls underneath. A worn lifter indicates the cam also has too much wear to reuse.
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!