I never watched Jack Kemp play, obviously, but I think rather than trying to work out his skillset translates (different game, different world) isn't a better question: "where was he ranked relative to his peers when he played?" i.e. was he a top 5 QB of his era? Top 10? Top half of the sport (appreciate a lot of his career was pre-merger)? Then look at how the modern equivalent for that level of QB gets treated by the market today.