Best braking...or good braking?
Best will cost you a bundle...say $1200-1500 by the time you get it laced up. (big Grimeca, Fontana etc)
Very good will cost you $500-600 laced up. (Suz GT750 220mm)
Good will cost you around $300-400 laced up. (Honda 305/450, Suzuki T500, Yamaha 650, Kawi h1 (early) 200mm).
This includes a spoke kit and probably a new alloy rim. Spoke kits go for about $80, decent alloy rim (Japanese or Euro, not Chinese) $150. Most of the time you can't use the orginal rim because the spoke holes in the rim are angled different since they are aimed at the small OD disk brake hub....when you throw on an 8" diameter drum hub the spoke angle is way different.
Might be cheaper to go with the entire front wheel of one of these bikes...assuming it's the same diameter, uses the same axle size, is not too wide. But then you'll have to deal with things like a different speedo drive, different brake stay arrangement etc.
To do it right it's a considerable undertaking. And you'll probably end up with a brake that doesn't work as well as a well setup single disk.