Another serious thing to consider is leaving that pond as is and just building another brand new pond in the location you want it.
You very likely may be able to build a completely new pond elsewhere cheaper than what it will cost to renovate your existing pond. Also you then know what you have as far as the dam being sound.
If an old pond is in a strategic location exactly where you want it the only option may be to clean it out. But if you have the option to build new somewhere else, you could potentially have two ponds for the price of one. The old pond could be your trash fish or kids pond (or forage or bait pond) and let it be what it is or have fun with it trying to manage it into something better. All the while the new pond starting out fresh the way you want it.
In my case I was able to manage my old refurbished pond with advanced size BG and LMB taken from my new pond after it became established. Buying advanced size stocking fish (to escape predation from fish already in an old pond) is very expensive and for some locations almost impossible to get. But if you can grow your own it is a pretty easy option. Having multiple ponds makes possible numerous more management options compared to only having a single pond.
my experience managing a pond older than me 
