clutch issues

I'm about to embark on trouble shooting what I think is my clutch since it doesn't seem to engage much and then only in low gear. Anyone have any tips on what to look for or where to best get a replacement clutch before I split it open?

These seem to be made of unobtainium. Sent my core to rebuilder only to be told it wasn't serviceable. I spoke with about every tractor and clutch company in the US. I found a Nissan unit for a 6-1/4" disc that I bought on eBay. I have no idea if it will fit, but am out of options to use the...