Hi Tim A couple of important points you don't mention.
How much sag does the beam have in the 24' span. All older beams support weight in tension...meaning the wood fibers (cells) act like tiny pieces of rope being pulled on from both ends. Also, how important is head room.
Unless you replace the beam with an 18" Glu-lam or 14X6" steel I-beam, the likelyhood is that you will require a support column in the middle for that much weight above it.
The other challange is that over the decades, the support joists and flooring above it have also dropped in tension and will require time to jack up and realign if you replace the beam.
If it were me, I'd get a residential structural engineer to come out and take a look at it, as there is a lot at stake with your house :-)