I like the idea of Kirk being a petrol head who rides motorcycles. It's obvious KirkPrime was a thrill seeker. He free-climbed rocks, vigorously road horses, practiced martial arts, and (as implied by original GEN cut) liked to sky dive from low orbit. I imagine George Sr. was probably some kind of outdoors person. (I like to think that the cabin in GEN was originally his and Winona's retirement place.)
It was clearly implied nuKirk's step-dad was a petrol head himself. Working on cars was probably the one good thing they shared. I know from my own father's experience, he pretty much hated his step-dad, but the one thing they bonded over was working on cars--which, of course, was something my dad and I shared, in-turn.
It's only natural that nuKirk, refocus that hobby into working on classic bikes instead.