You cannot buy M-14 parts, of any kind, from the U.S. All M-14 parts are restricted for export by their Federal government. There are no M-305 specific parts, Stateside, as Slick Willy(Clinton) banned the importation of the rifles back in the mid-90's. M1A parts are ok though.
A muzzle brake, not break, alters how the gases exit the muzzle. Usually by sending 'em up and back, thereby reducing the felt recoil. A flash hider doesn't really hide anything, but it spreads the muzzle flash out so it reduces the amount of flash seen by the shooter. If you've ever fired a firearm in low light(early or late in the day) you'll know how big the muzzle flash of even a .22 lr can be. Blinds the shooter. A flash hider reduces that. For us(most Americans too), one or the other is really just cosmetic.