Do you have the Haynes and/or Clymer manuals for these bikes ?
The oil in the clutch housing is most often the rear main seal. The manuals above describe fairly well how to replace it, but not perfectly well. It *could* be the seal around the input shaft of the transmission that leaked, but one can often tell fromthe smell of the oil (gearbox oil has a distinctly different odor from used engine oil). One critical thing is TO BE SURE TO BLOCK THE FRONT OF THE CRANK BEFORE YOU TRY TO PULL THE REAR MAIN SEAL OUT TO AVOID DISLODGING AN INTERNAL THRUST WASHER INSIDE THE CRANKCASE. Otherwise - you're in for an even bigger disassembly job !!!! Somewhere in our technical section we've got some pointers on this with a thread discussing someone doing it, with some pics.
With that many miles, I would definitely inspect the timing chain and sprockets, and check the tensioner as these have sometimes been known to self-destruct, break apart in high mileage engines.
Was this a running bike before you started breaking it down? Sometimes hearing it running can steer one into - or away from- areas that might/might not need attention.
It is good that you are going to check the clutch assembly. Inspect the splines on the transmission shaft and giver her a good cleaning and light coat of moly-based grease like the Honda Moly-60 stuff.
Chances are you may need a new clutch/friction disks but the diaphragm spring and carrier may be fine.
If you've got the engine on a bench I'd also pull off the pan and check the oil pickup screen , clean it all out and look for any particles that might lead you to other areas to investigate.
