I know this is an old thread, however, I had the same problem and after taking apart my ST4000 drive ring found out why the lever would not stay engaged on my unit.
Mine worked marginally if I held the lever in the engaged position and I tried the shock cord method of holding it there, however, it still seemed to slip a bit. When I took it apart the belt seemed fine but I noticed that the cam wheel with offset hole that attaches to the clutch lever spun on the shaft that connects to the clutch lever. It didn't move freely which is why I presume it worked somewhat. So I pulled it off, and sure enough there appears to have been two small pins which sheered off inside the cam wheel which would (if not sheered) cause the cam to be in a fixed position relative to the clutch lever. In the first two photos you can see the dimples where the pins sheered on the shaft and inside the cam. The second two photos show what the approximate position of the offset cam should be in the disengage and the engage position... either releasing tension off belt or applying tension to belt. In the engaged position the cam should be at such an angle that it will be held in position by the pressure of the belt.
Based on what others have said regarding finding parts for these old units, I think I am just going to attempt to drill a hole(s) for a new pin(s). This should solve the problem of the lever not holding in the engage position and also problems related to apparent slipping belt due to cam not locking into the fully engaged position.
1984 Catalina 36