For a bit over $200 you can get a (fairly accurate) repro complete caliper and MC from DSS. With new pads and lines, this gets you basically a new front brake... unless the disk is damaged, a deglaze is all it will need.
Braided lines are fairly cheap. DSS also has replacement stock rubber lines.
DSS made a big play on 400F restorations several years ago, buying every used bike they could find and turning them into "new". They found suppliers to reproduce unobtainable parts, and still have an amazing selection available. The 350 Four shares many engine and chassis parts with the 400-4.
Used brake parts are also available online or at scrappers since the 400F. 350-4, and CB350-G twin use identical components. Maybe others as well but the 350 twin was super popular and even now scrappers have bins of parts for them.