If you have added something original, non-obvious, and useful that improves the original invention, you may be able to obtain an improvement patent. But, beware: an improvement patent will not give you the right to manufacture and sell your improvement. To do that, you will need to obtain a license from the original patent holder (unless that patent has expired). However, it will prevent the original patent holder from manufacturing or selling your improvement unless it obtains a license from you. Often, the holders of the original and improvement patents will cross-license so each may manufacture and sell both the original and the improved invention. Patents are a very complex subject and you should not attempt to operate in this area without the advice of a licensed patent attorney.
The fall in new company starts does not necessarily mean that the American economy is less dynamic than it used to be, or that Americans are opening up new establishments at a lower rate than they did in the late 1970s.