I do. Even if he was adopted by his Indonesian stepfather, I don’t think that would negate his U.S. citizenship. He wasn’t of an age to renounce it and I think that if he was a citizen at birth, then he maintained it by returning to the United States as a minor.
The law in effect at the time Obama would have been adopted would not have precluded him from denouncing U.S. citizenship (providing he ever had it). Indonesian law didn't recognize dual citizenship, so it's within reason to think that Obama considered himself to be an Indonesian citizen. We know there's a school record from Indonesia that lists him as an Indonesian citizen, so the best available evidence tells us he was an Indonesian citizen. What we don't have is evidence of U.S. citizenship being restored afterwards.