Howard, read what's going on with your sweet ride, from what you described around the belt drive, as the build up of crud. That would suggest to be most likely the point of the seepage, being a shaft seal there dirt would stick to the oil and absorb it to the point it won't hold any more "hence a drip". I'd be inclined to focus on the seal area.
As a sudjestion, presher wash the area and whip it down with a clean rag (DRY) the best you can. Once done,..
If you have the means get the rear wheel off the ground, start the motor and put in gear, let it run for 5/10 min.
Shut it down and check around that area again. "COUTION YOU DON'T BURN YOUR SELF" on pipes. Your next thing is if it is a seal "why did it start to leek". You will need to check the shaft if there is a grove on the shaft surface
where the seal was seated on the shaft....if you do find a grove,(you may need a torch and mirror) to check the shaft. The torch you shine on the shaft as you look at the shaft through the mirror, this will bring up and help you see any imperfection on the shaft (ie)pitting or grove. If the leek is from there and you can not see any fault with the shaft, regardless rub the shaft down with a fine grit cloth (polish up). When re-fixing the new seal ("do not") re seat the seal in the same place as the old one. The seal dose not have to be completely bottomed out in the casing by doing this you are repositioning the new seal on a new unworn part of the shaft, a small bit of blue locktite around the seal body will hold it in place. External seals gather dirt, the dirt in-beds into the rubber of the seal then starts working as an abrasive and scoring in to the shaft then it starts to weep more dirt builds up to the oil more abrasive scoring to the shaft ....given time rubber could cut a steel shaft in half. Do hope it's something more simple. All the best