Hi Susan,
Do not know if this will help but there is only one James Mckenzie (it is noted down as M'kenzie) on the list that I have and the entry for him is,
(prisoner number) 2151 James M'kenzie, transported (no place given) on 31st March 1747.
For the John Mckenzie entries there are seven given on my list, one of whom died at Tilbury Fort Essex. Like the above entry the prisoner number is given first followed by name then what happened to them with the date of the event last.
Prisoner's by the name of John M'Kenzie
2157/2158/2159/2160/2161 and 2162 all transported on 31st March 1747 (no place given for transportation).
2171 John M'Kenzie, died at Tilbury Fort. No date given.
Not much but it may help as a reference point in the future for you.
OR.