Newt is also old, and not as solid.
Your suggestion of Mike Lee is a very, very good one. Lee is Cruz before Cruz went insane. He supported Cruz in Utah, but he has not been out there bashing night and day. He would unite the party. His downside is he is not as well known, and only helps in states that Trump will carry anyway.
Similarly, Duncan Hunter, Jr. is a great idea, but might not have enough of a profile.
It's tricky, because I'd like someone who helps in Ohio and Pennsylvania, but is not liberal. I can't stand Kasich. Now that I think about it, I think I've talked myself into Scott Walker. The GOPe trusts him, he is conservative on most things, and he is good in Ohio, PA and, of course, WI. Michigan, too, probably. Can he be trusted?
I think Walker could be trusted. Kasich IS a lib - big time, and I am beginning to think he is off tge reservation mentally.