On an Enderle/Hilborn style pump, the answer is "Nope". The gear, rotor and pump lid all have corresponding parts that must be the right size.
On a Waterman style pump, the wear plates and gears can be replaced to make a pump bigger or smaller, but they are a matched set as well to fill the space in the pump perfectly.
If you belt drive your pump, you could change pulleys to drive it slower. That would effectively make it smaller.
Spud
