Johann would be born in 1799 which matches what I have for the birthdate of Hillegien's father Jan Harm. He's listed with Gesina (assume 2nd wife)on the ship manifest at the right age and they are together in 1860 and 1870 census. All of their children are accounted for one way or the other except Jan who I believe died before the 1860 census. He is then buried next to Gesina (and Berend and in front of the Zaalmink plot) but his death date/age don't jive (since it says he's 70 and should be about 78). Lambertus and Hilligien are the only 2 children that I've found that don't list Gezina as their mother - they list Everdina. I have linked Alberdina and Everdina to Everdina also due to dates of birth when Everdina was still living.
The "other" John Mepjans is Johann's brother according to research I had done. He is buried in the Joldersma plot across from Jan Harm and Gesina and his death is listed at age 61 which would make his birth in 1812 which jives with Jan aged 45 on the ship. He and his wife Fanny are together in the 1860 and 1870 census (daughter Helen/Ellen who married Cornelius Joldersma) is listed in 1870 census but not 1860 even though all records show her birth in 1859. Fanny is in the 1880 and 1900 census with Helen. Her death certificate lists her burial here but I don't recall seeing a marker.
Just discovered several other siblings that lived in Drenthe, Netherlands - via his sister Harmina Mepjans Overbergen I found his nephew Lambertus Overbergen that came to America, settling in Iowa.