stockman, you don't necessarily have to replace the disk to fix the problem. Now that you know it is the disk, you can check whether it runs true. Just put it on the spindle, set up a stationary pointer clamped onto the mower frame, and rotate the disk, noting any variation in the distance from the pointer. If the disk's rim moves up and down relative to the pointer, you can take it off and bend it, then try it again, until it doesn't move up and down. Then, if it moves in and out relative to the pointer (radial runout), put a chalk mark where it is furthest from the pointer. Take it off, put a bar through the hole in the center of the disk, and rest the bar across two horizontal metal rails. Chances are the bar will roll along the rails a bit then stop. Make a different chalk mark at the high point on the disk. Don't be surprised if it is fairly close to the other chalk mark you made. Then use a bit of blue tack or whatever to stick a bit of flat metal plus two short 8 mm bolts, nuts and washers, on a convenient flat part of the disk near the chalk mark, as far from the center as possible. If the disk rolls and you end up with the chalk mark at the bottom, use a smaller piece of metal. If it doesn't move use a larger piece. Eventually you will find a piece of fairly thin flat metal that lets the disk be put in any rotational position, and stay there. Drill two 8 mm clearance holes through the disk and bolt on the piece of metal. When you've got it so that it is perfectly balanced (it will stay in any position when sitting on the bar across the two rails) try running the cutter system with it on the spindle.
Remember, you need that piece of metal to stay there, not be hurled into the distance like a loose mower blade. It needs to be securely attached. You could weld it instead of bolting it if you prefer, but that makes it harder to adjust after you weld it: you'd have to use an angle grinder, or drill some holes in the weight without going through the disk as well and making a colander of it.
