The wireless mechanical keyboards I have seen that used controllers from existing keyboards have been more like full rebuilds. The keyboard matrix has to be rewired to that of the controller of the wireless keyboard.
I believe your best bet might be this:
http://handheldsci.com/kb ... if there is enough space to place it inside your keyboard.
An oldie that was on Geekhack in the past (but is now banned..) once put this or something very similar into his IBM Model M to make it wireless. The Model M is quite spacious inside. Unfortunately, I'm afraid that the thread is gone, as I can't find it.
Another option would be a "Wireless USB Hub". In other words, it is a USB hub with no wire between the hub and a dongle that sits in the host. It might be even larger than the Bluetooth adapter, though.