Why not just buy a 20 litre tub of aluminium cleaner. Soak the cover in it overnight. In the morning the cover will be a dark grey brown colour. Chuck it in a bucket of water and scrub it and underneath will be nice clean aluminium. If it still has stains, just soak it again and then scrub it again.
Then once you have a nice clean cover you can decide if you want it shiny or not. And then proceed to sand and polish it smooth.

Thanks guys.
I always wonder what I would have done if I never discovered this site with all the good friends on it.
Will give it a go this weekend and post a pic of it fitted to the engine.
Cheers

Here is how ive polished my old L series rocker cover and all my alloy wheels(including the watanabes so check my galley for results).

Degrease the alloy
If its pretty ratty, use 800 wet and dry sandpaper(plenty of water)
Then 1200 wet and dry sandpaper (or if its not so bad jump straight to 1200grit). still lots of water.
If you wanna go crazy use 2000 after... but i never have.
Then get yourself a air compressor and a 'whizzy gun' or 'gasket scraper' or something like that.Something that spins fast and you can get a nice small felt pad for.
Apply autosol/other good metal polisher(dont need to use heaps) and spread over all of the alloy.
Attack it with the felt pad on the whizzy gun at full RPM.

Brings it up beautifully! Doesnt take long either!

i'd just have it chromed so that it'd be brighter and easier to clean up than polished alloy

Here is one in a KB10 belonging to one of our members. I believe it was photographed at the Nationals.

If the owner wishes to be identified, he can make himself known & thanks to Benny for the picture.
Coming from a front drive car, I'm guessing that the air cleaner was a bit cramped for room, so the slope on the side of these covers was probably to provide clearance for it.

Clearly these have shown up in Australia, so I am surprised that there are not more of them around on our beloved Datsuns.

































This is 1200GXmans new cover.
Interestingly, all three of these covers in this thread are on twin carb engines, but only 1 is on a street car.

Hopefully there will be 2 more in Aus soon. if 1200GX can find them for me.