I might try build one with a rigid fork and sus frame. I can always figure out the fork at a later date. I have a Fit s1 that I'm not doing anything with.
The BMW design was deliberately simple so as to keep the form factor of a regular bmx.
The fox air shock I fitted made a big difference to the weight and rideability. The BB pivot position meant it could bob quite a bit with the original coil shock. The air shock sorted that.
The fork was about as basic as you can get. :)
Although it did use a through axle long before most people.