Too all who have suggested many different conventional speakers - the OP has clearly stated that he likes the character of horn-based speakers. There are many conventional speakers that I think sound absolutely amazing, but I do not think any of them will reproduce the type of "horn" signature that the OP's Altec 19 will do. Horn speakers just have a different way of broadcasting the midrange and highs that no standard speaker can do.
I think it's going to be very difficult to get the same sound as the Altec 19, because that type of midrange sound really requires such a large horn. If you wanted smaller speakers, you could look into Klipsch RF7, which is just under 14" wide. It has a horn that is crossed over very low (at 1300hz). The Altec horn is crossed over at 1200hz, which is very close. The Klipsch won't exactly reproduce the type of midrange of the Altec, but I think it's probably going to be the closest you can get and still have a very small footprint.
The Klipsch Cornwall has a much better midrange horn, but the speaker cabinet is 25" wide, maybe not as big as your Altec, but approaching. Unfortunately, it takes a larger horn to get that big "throaty" sounding midrange.
The Klipsch speakers will have more high frequency response than the Altec. The Klipsch Chorus could be a compromise.
I think you'll just have to determine how much of a compromise you want with regards to horn speaker size. Smaller horns will not have as "throaty" midrange presentation.