The Ditton 15 is not exactly a reflex design, but used an auxiliary bass radiator (ABR) which is tuned to the box volume and bass/mid, so I agree that it is not a good idea to alter the box parameters too much.
Rather than changing stuffing, I may consider an internal brace or two between the front and back walls could help the boxes structural rigidity. These could start from between the tweeter and min/bass to rear and between the mid/bass and ABR to rear.
If they were cut so as to be a tight fit and then glued in place, they could be beneficial without changing the box volume too much.
With speakers of this vintage, I would also think about replacing the capacitors in the crossover.