I visited Srilanka recently and I visited many places associated with Ramayana. Chinmaya Mission in Ramboda, Srilanka is doing pioneering work with respect to Religious tourism in Srilanka wrt Ramayana.
During the trip, I saw many places associarted with Ponniyin Selvan. Probably Kalki had done research as many names are still existing in the same name or similar sounding name.
Probably, by doing more research, trekking /thematic tours can be arranged in Srilanka on Ponniyin Selvan Trail.
I do not know whether Kalki visited Srilanka. But his descriptions exactly matched the areas which I visited in Yala National Park (which is called Rohanam in P.S.)I felt like Alwarkadiyan walking through the dense jungle.
Kalki did visit Srilanka..not just once, I think thrice, before even he started writing PS. He has done his complete research and homework, before he penned the first word for each of his historical novels.