guiltyTicksLeft is the only field in Pawn_GuiltTracker, and it's private; not sure where you found lastGuiltyTick, since I can't even find it in ILSpy. =)
What you can do is call pawn.guilt.Notify_Guilty(int) with your own guilt duration, as it simply provides a direct setter for that value. For instance, this.targetPawn.guilt.Notify_Guilty() would make the pawn guilty for a game day, and Notify_Guilty(12*GenDate.TicksPerHour) would make them guilty for 12 game hours.
What you can do is call pawn.guilt.Notify_Guilty(int) with your own guilt duration, as it simply provides a direct setter for that value. For instance, this.targetPawn.guilt.Notify_Guilty() would make the pawn guilty for a game day, and Notify_Guilty(12*GenDate.TicksPerHour) would make them guilty for 12 game hours.