The absolutely best first move would be an aerator, not a fountain, nor a waterfall. There are many choices of course, but look here.
AES pond air A column of aeration will change the way the pond acts, making it a possibility to keep koi there well.
If you are having some drought though, now would be a good time to pump that baby dry. Empty it, dry it, then clean it out. You can give away your pike and bass but nobody will want the rest I'd think. If it was meant as a fishing pond someone will have put in a well meant Christmas tree. Take it out. Once you've got the bottom dry you can fill it. Find out about lime, I don't know enough, and add that to the bottom if you want to get yourself a good start like a "real" mud pond.
You could throw in some crappy koi to see if it will work, but remember that you are then stuck with crappy koi. Instead, continue to learn from folks that have similar ponds, KOI ponds, and go about building a collection of good fish. Remember that it is likely that draining and drying the pond will be a regular routine to eliminate the inevitable bluegills, to rebuild the soil banks and to replenish the bottom.
Good luck.