María,
I've not been able to find any birth record for your great grandfather on line. Assuming that he was born in or near La Joya (a distinct possibility), a personal visit or a conversation with the parish priest of the principal Catholic diocese in the area would be necessary. The church in Momax, the largest town in the municipality of Momax, is Templo Santo Tomás Apóstol, Iturbide #1, Momax, Zacatecas CP99720, México, telephone 55+ 43 7991 0026. The parroquia also has a Facebook page, at https://es-la.facebook.com/pages/category/Religious-Organization/Parroquia-Momax-Zacatecas-301343566865915/
Your great grandfather and his family were enumerated in the 1930 Mexico Census as follows:
Name: Roman Magallanes
Event Date: 1930
Event Place: La Joya, Momax, Zacatecas, Mexico
Residence Place: La Joya, Momax, Zacatecas, Mexico
Gender: Male
Age: 60a
Marital Status: Married both Civilly and in the Church
Birth Date: 1870
Birth Year (Estimated): 1870
Birthplace: Zacatecas
License Number: 6
Marital Status (Original Language): Casado por lo Civil y la Iglesia
Gender (Original Language): Hombre
Item Number: 4
Image Number: 461
Record Number: 16540
Household:
Roman Magallanes Head Male 60a born Zacatecas
Dolores Diaz Wife Female 38a born Zacatecas
Francisco Magallanes Son Male 22a born Zacatecas
Victor Magallanes Son Male 19a born Zacatecas
Ramón Magallanes Son Male 17a born Zacatecas