Agree with the first two replies.
I have had experience cleaning out three old ponds and building
three four new ones doing the work myself or with the help of some of my hired farm hands running the machinery. I actually did all the clean out of the old ponds myself.
I can tell you from experience, building new is a lot more "fun" from running the equipment side than cleaning out old. Moving muck (usually multiple times) is NO fun.
Often times when people come on here posting a site with an old pond asking about enlarging/cleaning it out, I look for adjacent areas to build a new pond instead. Unless the old pond is just in a strategic location (like right out your front of back door) I suggest leaving the old pond "as is" as a trash fish or kids fishing pond and just build a new pond somewhere else on the property. It is often much cheaper, quicker and better to build new than refurbish old.
Here is one of my three experiences of cleaning out an old pond. And in this case the pond was so old it was almost completely dry. Almost.
Reclaiming a 50 year old pond Edit: I do not own an excavator. An excavator is almost a must in a pond renovation so hiring the right experienced crew with the right machinery would make all the difference in the world.