Have you checked with Westlake? Have you read the specifications for the drivers? I'm rather unfamiliar with what is inside Westlake monitors -- but if you read the driver specifications you should be able to land on an "after-market" driver that is equal to the efficiency of the "factory original" driver in the cabinet.
Matching the efficiency of the driver is of paramount importance in order to maintain the balance between woofer and tweeter post crossover -- a less efficient driver will net less bass in the resulting response, a more efficient driver will net more bass in the resulting response -- neither of which will give you the same response you had before changing the driver.
Best of luck with your search.
Peace