TKOP, this one is not as simple as it sounds...that tube is vented ("ported"). You will need to find drivers that have the appropriate parameters to have this thing work decently. Just choosing the right size and impedance won't cut it. If the only thing wrong with the current drivers is rotted surrounds, you might try replacing them with kits available from Parts Express.
Have you ASKED Parts Express, MCM, and/or Madisound to recommend suitable drivers for this enclosure?
BTW, is the 4 ohm impedance you mention for the system? I.E., are there two 8 ohm drivers wired in parallel?
Finally, Madisound has some 10" Clarion car subs for $25 each. They work great in 0.5 cu ft sealed boxes. Note that they are 2 ohms; make a 1 cu ft box, drop 2 of them in there and wire in series for the 4 ohm impedance preferred by most bridged car audio amps.
www.madisound.com
Click the "on sale" link