Pete,
OBD-I stands for On Board Diagnostics I. This was a government mandated emissions program that was required in all cars. The requirements for these emissions programs changed significantly in 1996, OBD-II. Any new car sold in the US since 1996 was required to have this emissions program.
It basically monitors the cars emissions, and causes the Check Engine Light (CEL) to illuminate when the emissions, or a sensor is out of spec.
Being that the Contour was sold from 1995-2000, your contour is the only year that had OBD-I.
Additionally, our cars have secondary ports that open around 4K RPM. You probably felt a surge in power when accelerating beyond 4K. That surge was the secondaries opening. On 1995 V6 models, the secondaries were controlled by a vacuum and are open when the car is off. Since 1996 they are cable controlled, and closed when the car is off.
Now back to your original question. What engines are available for your car. Without major modifications, your best bet is probably another 2.5L V6. They are a great engine, and hold up very well for the most part (when the water pump doesn't fail! - Common Problem)
Additionally, you can drop in a Duratec 3.0L engine. These engines are found in the Taurus, Escape, and a few other cars, and will give you even more power than the 2.5L Duratec. However, I don't believe this is an easy swap for those people with OBD-I, like you and others with a 1995 Contour. This doesn't mean that it can't be done. In fact, it has been completed. Be sure to check out the 3L Forum in this group. I'm sure they'd be willing to answer more of your questions.
