One of the ways that legal scholars try to figure out the original intent of the constitution is to look at how that phrase was used in other documents from the same time period. Even better is other documents from the same time period that are written by many of the same authors. The only significance of the 1790 act is that it was written just a few years after the constitution was ratified and it had many of the same authors.
So, if the congress of 1790 which included many drafters of the constitution thought that children born to U.S. citizens outside the U.S. could be considered natural born citizens. Then it is likely that many of them had a similar view of what a natural born citizen was when they included the term in the constitution.
You assume a lot. Number one- There is only one condition of citizenship that is universally recognized and therefore never requires a statute of any kind: born on the soil, of two citizen parents. Every other condition requires statutes to establish citizenship. Natural-born seems pretty simple to reckon from that simple methodology. Obama is therefore not natural-born. Oh wait...