Robinson R-44 pilot reported a loss of power while conducting a sightseeing tour; resulting in a precautionary off-field landing.

Narrative

Loss of power and consequent precautionary landing.I was flying at 500 feet on a Robinson 44 on a sightseeing tour north of the State Park; over the river; when I felt a loss of power. I decided to reverse my course from northbound to southbound and do a precautionary landing on an open grass field in the park. After smoothly landing on the grass; we exited the helicopter in a normal way. There were no injuries or damages of any kind. I contacted the company that sent a second helicopter to pick up my customers and drop a Mechanic. When the second helicopter arrived; it flew back to ZZZ airport with the customers and the Mechanic started to look at the engine. Shortly after he determined that the push-rod on cylinder number 2 got bent and the exhaust valve got stuck.Another mechanic arrived at the field with some replacing parts and engine oil. They replaced the push-rod and we added 3 quarts of oil that were lost due to this failure.Before the closure of the park; the first Mechanic and I flew back to the airport without any inconvenience. What I have learned is to always have in mind spots where to land in case of need; plus; I will fly higher than 500 feet when possible to have a better glide distance to reach any safe area.