It has been years since I have gone through a BW 1356 but I am going to venture that you need to see the output shaft and see if you can put a drive gear on it and then get an extension housing that has the hole for the speedo cable sensor.
I figure that, at the worst you are going to have to get the right output shaft that has a provision for the drive gear and the right drive gear for your rear axle ratio and gut out the TFC and put the shaft in it and bolt on the extension housing.
I would get the fork pads to put in there while you are at it and check your seals and possibly replace them, too.
Not too hard of a job.
I got confidence that you can handle it. Make sure to use a really good RTV sealer. I used to favor the goop that was called 'The Right Stuff'.
Good luck on it and maybe you can send us some pictures?
Take care,
X