Same as any other rust. Remove all the rust with a grinder, wire brush, sandpaper, etc. Weld up any major pinholes and grind flat. Then its all refinishing work. More than likely finding a replacement is the best method. The sunroof rust from the inside out, so there is most likely a lot of rust towards the bottom. Its not too uncommon to see rust all the way around the sunroof that hasn't popped to the surface yet.
Be Picky when cleaning the rust from the edges, you might have to use a sand blaster to get in between the thin layers of metal around the edge, and if you remove the sunroof chance are you will need a new seal ~30 bucks from Toyota.
I just finished de-rusting sanding and priming mine....I had to go through 6 damn parts car to find a sunroof panel that was able to be restored...the sunroof seems to like the rust in was that you just can't fix.
But one the good side.....The celicas of the same years (82-85) have the same roof panel, and they are pretty common.