Besides the couple of things you are already looking into: pumping and plugging/stoping your leak...
Try and maximize the runoff area you have. See if any small berms could be built to direct more water to the pond. Maybe pipe your rain gutters to your pond (helps more than you think). Depending on the area, you could add some clay on top of the top soil. And last but not least, a big circus tent over the pond to reduce the evaporation
Nice pond BTW.
