If you haven't already, get a 5mm allen, take off the drive sprocket cover. examine the drive gear, and all that. I have noticed that many actuators are showing a wear grove from the one hole, to a line to wear it will break though and no longer work.
2, with that part disconnected, pull apart the handle bar segment. the clutch adjust on the handle bar should have a wire remove/replace grove, it just needs to be aligned . examine the cable, where the wire goes into the shealth, and ensure that there is no fouling of the cable.
3, provided everything is good, put the handle bar back together, put the drive sprocket back together, tighten up the handlebar side. revisit tests.
as far as your brakes, your going have to refer to the manual. I don't know anything about front drums. My immediate thought is that the spring, has sprung, and your going to have to open it up, and re-clip it. If you haven't downloaded the manual, there is free copy on top of the garage.
The next link, will have the front tire breakdown.
http://www.cmsnl.com/yamaha-xs400-1981-b-usa_model8825/partslist/