Islam is a religion, not a country. So if a country chose Islam as a religion, the ruler's title would depend on the country's politics. It could be president, king, etc. In the past they used the equivalent of these titles in Arabic like Khalifa or Sultan which they both translates to ruler.
If meant by Islamic sovereign the ruler, then is named Sultan, king, president, amir, or as called before Caliph If meant Islamic religious leader, then is named Imam or Sheikh
An Islamic ruler is the one known as "Khalifah" which establishes the suprmacy of Shariah and Islamic state known as "Khilafah" in the world which covers all the Islamic countries of the world and establishes ONLY one Islamic state in the world ( From Morocco to Indonesia).
Imam.
General Zia Ul Haq implemented some islamic laws in Pakistan .
Emirate
The Islamic Empires had a heredity system of succession of rulers, which meant that rule was passed to the children of the former ruler. However, if the ruler had no children, then there was much dispute and fighting over the throne.
A Caliph or khalifa is a sort of religious dictator, an Islamic ruler, considered to be the political-religious leader of an Islamic community of believers, ruling in accordance with Islamic law.
a ruler is called "Fürst"
A ruler is called 'une règle' (fem.) in French.
Called a straight edge
The Islamic guidelines for righteous living are called the Islamic law or Shariah. Refer to related question below.
A egept ruler is called a Pharaoh