hmmm...
It's been a long while since I worked on one of these...But as I recall, the headlight cover is held on with a roll pin. The pin provides tension which allows the cover to stay up. It may be worn. Therefore, you'll need to knock it out with a long punch pin that's the same (or slightly smaller) diameter as the pin and then install a new one. I don't think there's any way of doing it without removing the fan case first. I always used to wrap it in cloth and clamp the fan case down in a vise in order to stabilize it. Care must also be taken with the lugs on the fan case, lest you break one while knocking out the pin.