I am not aware of any diagram that shows the proper route for the cable. Try to find a similar year bike and look at it.
But routing the cable isn't rocket sceince. Avoid shart bends. Like others said, I highly recommend a new cable.
Suggestion: Get a new cable, and hook it up without routing through the frame, just as a test. Keep the bend as large a radius as possible and then see what the clutch feels like. If it gets significantly worse once the cable is routed then it is a good indication that you have too tight of a bend.
I would not expect the lever to make any signifcant difference. If your lever is too short then it could be the problem, but I still suspect the cable.
It is also possible that the problem is in the cluch itself. I bought an after market clutch for one of my bikes once and it came with new springs that were a LOT stronger than the original. I hated it, and eventually went back to the OEM clutch/springs.
As for the levers: I am a fan of the ASV levers, although as I said above I wouldn't expect the lever to make a difference in the clutch operation. I have put ASV levers on all my bikes and have never had to replace one. Before I started using ASV levers I would go through the stock and cheap afer market levers like crazy.
Rod