I used a clutch assembly from a New Holland Super 67 baler to replace the junk aftermarket bush hog clutch that was on my 520 baler when I bought it. Seems previous owner must've had clutch failure. 

The clutch bolted up to the factory holes, I just had to add a few washers to space it from the fly wheel mounting plate. I can post up pics. 

If for some reason what Chris has doesn't work out, this is an option that works. 

I do know the parts are still available from New Holland new (but $$$).