Actually Val's idea sounds nice, but did it ever over flow when the Koi were still in it.
Find out what is under neath the sand, take a steel rod, of some sort, and hammer it in the bottom of the pond.
If it goes in , then, a post hole digger to make severl drainage holes to totally drian it is the best idea.
It looks that whom ever made it did a good job, which means the soil under the pond was probably compacted so there would be no drainage if the liner was punctured some how.
It is a good idea of what is under the bottom sand before you start filling it in.
