Do Modern Aztecs still practice religions from the past?

Some, yes, but the majority are Catholic. The Spanish arm of the Catholic Church was extremely aggressive in conversion, even starting an American branch of the Inquisition. Often, elements of the Old Religion are found in some Catholic rites. Most Mexicans are have both European Spanish ancestry and Native Ancestry, but, some full blooded Aztec and Mayan decendents do indeed still exist and since the mid 1990's there has been a growing movement to re-embrace the old culture and religion.