He contacted genealogy experts, the hospital where the baby was delivered, the Ministry of Tourism in Tonga and the Tonga Registry Office, even posting on Facebook, before finally getting the name of the girl's village, Ta'anea, from the Tonga High Commission in London.
Dr Clark, who worked as an A&E consultant, was 26 when she was sent to train in Tonga by her London medical school in 1997, and wrote letters to her parents recommending places for them to stay.
She told her father on her deathbed: "Dad, I don't want to be forgotten."
The trip, from December 15, 2018 to January 17, 2019, saw the couple travel from Chandler's Ford, Hants, to Macquarie Island near New Zealand to photograph rare Royal penguins, to Northland, and the Tongan Island of Vava'u, where the baby was delivered.
Writing about the delivery in her journal, Lisa said: "I went to the hospital and after a lot of waiting around I delivered my first baby and it weighed 9lb 12oz.
"It was all a bit nerve wracking but I loved it. It was the mother's eighth baby - she was 40 - and it was a little girl this time.
"The lady was very proud though and even named her after me."
After a visit Ta'anea hospital where the child was born on January 28, 1997, a village resident recognised the mother from the Clark's photograph and directed them to the family's house.
We had followed Lisa's footsteps though her diaries, we had seen what she wanted us to see
Roger Clark said that, after a halting introduction, they had managed to communicate with the Tongan family via one member who spoke English, eventually being introduced to the 21-year-old girl, Meleane.
"I really felt very humbled to meet the family," said Clark. "I knew, for me, it was a really important moment and I wanted to make this girl feel special, but I thought about it from her angle - we were just some tourists with some loose connection to her.
"We had to prove to them we were friendly and honest, and we ended up having a wonderful connection with Meleane and the family."
After dinner with Meleane, the couple made their final stop at Paradise Hotel in Ta'anea, which Dr Clark had stayed at during her trip.
"As I slowly shut the door to room 115 [where she had stayed], I felt a strong sense of personal closure," Clark said.
"We had followed Lisa's footsteps though her diaries, we had seen what she wanted us to see, we had touched on her foreign experiences."