Even until now it is still rather common for Indonesians to have only a first name. Another example is Soekarno, our first president, who felt the need to add another name Achmed in front of his name because when he visited US, some journalists were asking him his complete name, not realizing for Indonesians having only a one-word name is common thing.

Of course, many people at the time also had a birth name, which may or may not be different from their legal name. Soekarno himself was named Koesno when he was born, that was changed to Soekarno (taken from the name of the hero Karna of the epic Mahabharata) when he was five years old. Fatmawati, Soekarno's first wife, was named Fatimah when she was born.

As far as I know, Wilopo was included into the 'one-word name' category. Even if he had a different birth name, it had been lost with history.

Also quite common for people at the time, especially royalty, was having several names other than birth name and legal name. Raden Mas Soewardi Soeryaningrat was a royal (the ningrat part from his surname indicated his royalty, and you could safely assume every Javanese with a ningrat or ningrum on his or her name is somehow a descendant of a Javanese royals, while the Raden Mas part is his royal title), but he is more well-known by his humble alias Ki Hajar Dewantara.

Name changing has many reasons. In Soekarno's case, he was sickly as a child, so much that his parents felt the need to hold a special ceremony called ruwatan to change his name into a more fortunate one. Others, like Ki Hajar Dewantara, wished to be closer to the laypeople, and therefore he threw away his title and birth name in favor of a more humble name, signifying his readiness to fight together with the laypeople as one, not as a superior royal. Some would have their names changed from received wangsit (a kind of divine inspiration) either from God or from revered ancestors.

Naming process wasn't simple; it was a very, very complex process, very unlike parents nowadays who give their children fancy names without meaning or taken from foreign languages. Every name must have a special meaning and every meaning was given as an embodiment of the good wishes and prayers of the parents as well as the relatives of the child. Sometimes, a witch doctor that helped with delivery would choose a birth name for the child that signified how his delivery process went about. Every name was chosen very carefully either from a good-meaning word from Sanskerta or Javanese, or more commonly for laypeople, from wayang heroes which stories usually taken from the epics Mahabharata and Ramayana. The more unfortunate ones may even receive their names from an acronym of what happened during delivery process, such as the name Sukirno, which is an acronym of esuk-esuk ora ono (early in the morning and he's already gone). God only knows what kind of happenings would prompt the parents to give such a name.

P.S. This one-word naming thing does not apply to all Indonesians, due to the difference of culture. Javanese don't really know any concept of family surname, but for Bataknese, surname is taken from the father's line. Balinese also don't really have family surname tradition, but their given name usually has at least two words, leading to confusion for some foreigners who wonder why the hell do Balinese have only Made, Ketut, Nyoman, Gde, Nengah, and Wayan for their given name while the surnames are bountily varied.

Starting from early 20^th century, though, European naming style was becoming popular, so much that some families even invented a family surname. Example is Susilo Bambang Yudhoyono, our last president before Jokowi, whose father, Raden Soekotjo (another one-word name holder) invented the Yudhoyono surname and gave it to all his children.