The 860, 970 and 1180 are in the same manual and similar.
The drawing for your machine is here:
http://bsintek.basco.com/BriggsDocumentDisplay/default.aspx?
filename=heCBEN8brF5rE6CGk5kbp796Dq
It looks like you would have to separate the bucket from the tractor to access the clutch rod and break assembly (31). There is the following note related to the spring (36).
Hook one end of spring (Ref. 36) over clutch rod assembly (Ref. 31)
between tabs and other end into slot in frame. Right rear bolt
Position belt stop (Ref. 42) 1/16 to 1/8 inch from belt when auger clutch is
engaged. Drive Group. idler arm (Ref. 25).
I’ve had similar clutch problems with mice nests in the tractor making the clutches hard to engage. Changing the belt you would have notices one in the front but there could be one in the back corner. You could flip it up, remove the bottom cover and look around. Sticking parts under the dash or linkages could be hanging up the clutch travel. If you were sold a belt that was too short then that could also cause the problem. You could remove the belt and check the ease of travel of the clutch rod and work back from there up to the lever.