Boosters are hard to get. Good luck finding one that fits at any auto parts. I lucked out and grabbed a new one (Bendix type) for my 65 Newport on rockauto for $70 (clearance). Usually, you must try somewhere like "Booster Dewey" and pay $300. I have not seen rebuild kits. If desperate, you could drill holes to fit another booster to your "booster plate". I had a Midland-Ross booster from a 74 Dart that looked just like the one (replaced once) on my 65 Newport, but the bolt pattern on firewall side was different. I drilled 3 holes and made it work, but one hole was in a rippled area. I later got the correct booster (above) and sold the Dart one. I also tried fixing my original M-R booster. I bought neoprene sheet (ebay) and made a new rubber diaphragm (mine had a hole right @ vac port, though I had used a fuel vapor filter). It works when tested off-car w/ vacuum, but not confident the rubber will last long (haven't installed). Another data point, the booster on a 95-99 Breeze or LH cars has bolts real close to a Dart, since I mounted one on Dart stand-off brackets (photo above) to use on my 65 Dart w/ just a little rat-tail filing. Indeed, they are closer together in width so might have fit better on my Newport's plate. If willing to use A-body stand-off brackets on your C, you can buy a booster w/ MC & brackets for ~$170 on ebay. It uses some GM parts. That would help insure you clear after-market parts (valve cover, headers).
Once you have a booster, the MC is easy. Any 4-bolt Mopar MC will bolt up. If you dont like rust and weight, use any 2-bolt MC w/ 2-4 bolt adapter ($30 ebay). Many use an 80's Dodge truck MC. I used a 95-99 Breeze w/ ABS MC on my 65 Newport's booster. Disk/drum MC's have a larger reservoir for the fronts (aft of MC) or later MC's have a shared reservoir. Bore size is preference. 1-1/8"D (above) sounds large, but depends on the caliper too. Most A-body guys use 15/16"D for an easier pedal (at expense of more travel). My C-body has a 7/8"D and the pedal barely travels. With the booster it stops "authoritatively" w/ drum brakes, but would be about right w/ front disks. Insure you install a proportioning valve in-line w/ rear drums or the rear will lock-up first and cause the car to spin out. Adjustable ones are just <$30 on ebay.