Bob Wischusen was my first thought as well. He's already well-known in NY sports media (he's the radio voice for the Jets). I enjoy Bob's PBP on ESPN (he should be the #1 instead of McDonough there), and think he would be a great replacement. If not him, then I agree with other posters--Brendan Burke or Howie Rose would both be great choices. I think Kenny Albert has too many other duties to take the NYR role full-time, being the #1 PBP guy for TNT, but he would also make sense.
Sam is a legend, and I hope the Rangers do him right with a proper ceremony/celebration for him throughout the season. They really need to borrow JD from Columbus to bring him back in the booth with Sam for a few games, for old time's sake!