That back brake is pretty powerful on its own! Change to a braided hose and perhaps aftermarket pads and I reckon it will be man enough for the job. If you change to a later swingarm, wheel and brake you can get a 4-pot caliper, but on a smaller diameter skinny rotor without the heat sink capacity of your current iron one.

 

Man Beemer that is beautiful. Why would you want to hack that up. I would think a beer keg with an axle under it would be more appropriate..

 

Really sweet looking side car Beemer!

 

Oh, so that's a hack.

Beary