Welcome to the forum!

Quickest and easiest would be to drain it and start digging. You could make it bigger and deeper while eliminating the vegetation. I have a similar situation but with a smaller pond. This is what I would do if I could spend the money on it. Just do a lot of research on building/renovating a pond before you go crazy with the backhoe. A hole in the ground is pretty easy to get. A good pond isn't and you might do more harm than good.

I can tell you from experience that cattails are tenacious. The herbicides that many people on this forum will recommend are restricted in NY.

The following is a quote from the DEC when I asked about this subject: "Cattails will likely need to be pulled by hand or excavated with machinery. If you want to use an herbicide, you can hire a certified applicator to use an approved herbicide. See the DEC website at: http://www.dec.ny.gov/chemical/8530.html"

So now that I've dished out a bunch of doom and gloom, what are your goals for the pond? Is 4.5 feet full pool depth or is it down a couple feet? Are there fish it it? Do you care about fish?