Bass speakers for 113 liter /4 foot cube closed box

I am helping a neighbour repair a three way speaker of danish design from the 1980s. The bass speakers do not work. They are are unmarked, with a cutout diameter of 23cm=9 inch The cabinet has internal volume of 113liter or 4 cubic feet.
I would appreciate tips on what spec's of speaker I should be looking for.
From testing frequencies I have established that the midtone drops of at 100Hz