There are plenty of KEF B139s on eBay. Many were made, and are still in very good working condition. I have six in regular use at the moment, and some spares.
The KEF B 139 has the most perfect T/L parameters for TL loading and was the basis for many. Jim Rogers was part of the Radford group, that did the really ground breaking work on TLs. Raymond Cooke had TLs in mind when he designed the B139. There really is no adequate replacement for it. In any event using a different driver, apart from miss alignment, would require a complete crossover redesign.
There are two for sale in eBay right now.
The KEF B 139 is a really special driver and certainly in the top five of all time. So you need to pony up and snag those drivers. Anything else you could do, would be way inferior than replacing with the genuine article.
Two B139s in their old location.
These speakers in our new home. They are the rear backs, in my old and new AV room. They used to be my reference monitors in my former studio I did a lot of mastering in and edited radio broadcasts.
This is those dual TLs in their current location.
Here is a B139 in a three way B4 design.
This is the front of our new AV room, with full range triamped dual TL mains, and biamped TL center.
In wall TL for my wife's great room system.
The take home message is that you need to use the B 139 drivers, those excellent lines were designed for, and not try and substitute. The only alternative is a totally new design and build.