If you can stand the thought of running riceburner parts, there are several small-diameter boosters on Jap imports that can be shoehorned into the frame rail area near where the original M/C mounts, and operated with an offset pushrod. I'd suggest adapting a master cylinder to the booster that's more volume-compatible with whatever brake system you're planning to run, as the Jap rigs run some very small-diameter M/C's. It would take a little fabrication work, but it's doable with a little tinkering.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!