If you are looking for speed you will most likely need to change out the entire drive train. If you just want a little more speed I would either change out the rear end to a more modern with a higher ratio or add an auxiliary transmission.
While a little more expensive than finding a used older auxiliary you could easily add a gear vendor over drive (
http://www.gearvendors.com/agmanual.html). If you change out the rear axle you may have an issue as your wheels won't all be the same. If you change out the transmission....then what is the point in keeping the original engine as well.
If you do change axles make sure you try to sell it....there are a lot of people who would for sure be interested in an old two speed...depending upon location and price of course.
