It's really easy! *
First you buy a Crown XLS-1002 amp. At $370 it's a small price to pay to test if the old sub is even working. Never mind that it's more than half the price of an entry level SVS or Hsu 12" sub.
It's all about the science.
If the Wharfedale sub is ported, you just run the speaker wires through the port to the driver.
If the Wharf is sealed or passive radiator, you simply drill a hole into the cabinet, run the speaker wires to the active driver, then seal and caulk the hole. This is completely reversible with some bondo and vinyl that matches your sub's finish.
Set the Crown amp for a suitable bandpass (read the manual, don't ask me how), adjust the gain to match your system, and let 'er rip!
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* Note 1: OP, PLEASE don't do anything I just said.
Note 2: If I could flag my own post as Dumb, I would.