Thank You Linnea,
You got me looking in the right direction. New York Times and Arno Press put out a series: "Eyewitness accounts of the American Revolution" in 1968 which includes some 60 primary sources including Mackenzie's diary and that of another member of the 23rd Regt--Sgt. Roger Lamb. These are the two gems I've been looking for and they are available through interlibrary loan.
Mike Davies, Private Soldier
Recreated 23rd Regiment of Foot
Royal Welch Fusiliers in America
To Join the Company of Military Historians click here