The 67s were introduced when the Mk3s went push pull, prior to that they ran top and tail. Unless the 57s could be made to Work push pull and with Mk4 stock including bell-buzzer and crew-loco communications then its a non starter, even ignoring the top speed.There must have been a good reason at the time for changing to 67s- higher top speed? Is there anywhere on the route to exploit this?
The comparison to the Night Riviera is a totally different type of work with just 1 one way trip a day compared to 800 mile diagrams for the 67s with both start-stop and high speed 110mph running on the same diagrams.
The chances of any loco swaps at this point are slim but those to anything older, slower and not modified to work on the stock their hauling is almost nil.