Disc brake setup on YBZ is possible. You can take the example of Kawasaki GTO 125, Honda Deluxe. I have already done myself as DIY on my Suzuki Sprinter.
You need atleast, brake booster, pipe, calliper, disc and drum. This setup (matching YBZ) is easily available in the market.
Secondly you can fit the calliper on the current jump bottles by fabricating customed made braket. Otherwise proper bottles are also available matching the diameter of YBZ front rods.