All I know is he died on 20th March 1837 in Sydney and was buried in the old Sandhills Cemetery. Just the bare details, name and death date is all I have for him and that was from the Gravestone Inscriptions NSW Volume 1 Sydney Burial Ground: Elizabeth & Devonshire Streets - "The Sandhills".
His marriage certification is very bare, only his name and wife's name.
That is why I am 'stumped'.
Agh

so you have documentation for the marriage and the burial and those two events occurred BEFORE the commencement of civil registration in NSW, and thus from the era BEFORE the civil authorities required fuller details. Burial records from pre 1856 are so scant with the details. May I ask how you know your chap changed his name to JOHNSON ? Have you considered that it may be a pointer to his likely/possible maternal line?
I guess you have searched through the family history section of the NSW State Library and also perhaps know your way around both the NSW State Records Office and SAG as well
When/where did he marry, and what name did he use when marrying? Have you got to the actual parish register or have you needed to rely on the transmitted records from all the various denominations held at St Phillips as per Governor Macquarie's General Orders from Sept 1810 ?
I do sympathise with your quest, and I definitely know why you are 'stumped' at the moment.
Cheers, JM