This is one case where a bigger Star Actor would have helped
Usually I'm all for the creative forces behind the film picking who they feel is best, but in the case of The Rocketeer, I feel Disney was right in that a bigger star, perhaps an A-Lister would have given the film the right boost it needed. I'm trying to think who in 1990, 1991 would have been a good A-lister for the role. Tom Cruise? Cruise would have been a huge draw in 1991. Hell even Tom Hanks would have been good, Hanks hadn't gotten his Oscars yet, but he was a more known name then Billy Campbell. If they had wanted to go younger in 1991, River Phoenix was still around and about 20 or 21 years old at the time.
The whole cast was nearly a bunch of unknowns in 1991, which I think hurt this movie. The only one I recognized at the time was Timothy Dalton as the bad guy, only b/c he had starred in the most recent James Bond films from the late 80s. So that's like today casting a movie with Daniel Craig as the villain, and the rest of the cast are near unknowns or at least, not big names in a major Disney fantasy adventure flick.
My picks are the two Toms. Cruise or Hanks as The Rocketeer in 1991. I guess you could keep Jennifer Connelly in the role, but Meg Ryan was also around at the time.