Once rusty, the top inspection cover is hard to restore. I started with media blast and worked my way up from 80 grit to 600 on a DA. David Kee transmissions sells a nice (they say) cover that only needs to have a vent hole drilled.
Ebay has the proper gaskets now and again or PM me since I forgot I had one and found another, so I have one to spare.
See my build thread for a restored example of the gasket and top cover/writing.