Here's an accessory starting with a special request from someone who dearly wants one exactly like this. This is a VERY period aftermarket accessory alloy valve cover for the MGA. Notice there is no external flange on the bottom and large radius on the bottom corners. Instead it has thick sidewalls, so it will be moderately heavy. If anyone has one of these they could part with, please contact Del Rawlins at del@(remove-to-reply)rawlinsbrothers.org.

On 07 November 2009, Del Rawlins in Alaska wrote:
"I'm restoring my dad's MGA, and am reusing the cast aluminum rocker cover that he put on it sometime back in the mid '60s. It isn't like the ones that are available today, and it is kind of a pain since I have to re-polish it, and make a special gasket since the standard ones don't fit right. But, I want the car like it was when my dad had it, so it's worth doing, at least to me.
Now, my brother also has an MGA project, and he's expressed an interest in making it look like dad's. So, I'd like to find him the same rocker cover, if possible. The only other one I have ever seen, was in a picture of the "MGA Hawk" racing special (the one with the studebaker grill) on Barney's site. But, where there were at least two made, there were probably others, so I am hoping that somebody here will have one sitting in a box somewhere that they would be willing to sell me. You'll probably be doing yourself as big a favor as you'd be doing me, since as mentioned, the standard gasket doesn't fit right, and also there is less interior clearance for the rocker assembly than the stock cover has. I'm attaching a picture. I know this is a long shot, but thanks for looking and reading this far".

Here are more examples of alloy valve covers that work on the MGA (or any Austin B-series engine).

Moss Motors current issue

Valve cover with oiler for
use with Judson supercharger

Older Moss Motors cover

For this one the eBay ad said the vent tube was added.   After some internet chat, within a day or so a few more people report having one like this in their posession. One says "Moss" on the bottom, so apparently they were sold by Moss Motors some years earlier. Not particularly rare, but still quite nice.

And the plot thickens when another one very similar is found with no "Moss" name inside. This one has a hole where the separate "MG" emblem is screwed on rather than cast in place. Also notice method of attachemnt for the oil filler cap, as the standard cap will not fit this cover. This appears to be a vintage Ford oil cap. When there is no valve cover vent port you will need a vented oil cap, the tall one with horse hair inside to serve as air intake filter.

Bill Spohn in W. Vancouver, BC, Canada wrote:
"The appropriate valve cover for a crossflow head is the Derrington cover".

I suppose that's a matter of opinion, as not all cross flow heads are Derrington brand.

Knurled thumb nuts for the valve cover. Still available if you look around a bit. Strong finger torque is enough to seal the gasket if the cover is flat on the bottom. Extra length nuts are also available for the thicker alloy valve covers.

Addendum July 2011:

I have seen a few copies of this one, although I do not yet know the source. It looks a lot like one that was on my MGA when I bought it in 1977, and I drove it with that cover for several years (1986 into the early 90's) before trading it for a stock sheet metal cover. Having no valve cover vent pipe, it must use a vented filler cap. The cap is full of horse hair or wire wool and vents through multiple holes in the under side (which will dribble a little oil on the valve cover). The coupling in the cover is a round hole with two square notches and cam ramps inside. The cap has two radial fingers on the bottom that go below the cover to engage the cam ramps for twist on. When I needed a flat washer style gasket for it someone finally told me it was a period Ford oil cap. He also pulled an identical cap off of his vintage Ford pick up truck (with sheet metal cover) to give me the thick paper gasket (nice bloke). That alloy cover has long since found a new home, but I may currently have a lead on another one to find the brand name.

Also see article AT-126A for vented oil filler caps that may commonly fit these aftermarket alloy valve covers.

Addendum Jan 16, 2022:
Here is another one from Giovanni Delicio in Obrigheim, Rheinpfalz, Germany. This LANDAR valve cover come with an MGA 1500 engine purchase (sitting on an 18 head).

Landar was a kit car manufacturer back in the day. An alloy rocker cover was produced specifically for use on a "Landar" kit car. This rear engined Mini derived racing car (A-series engine) was produced in small numbers towards the end of the 1960's. This rocker cover fits a B-series engine, so they either graduated to larger engines or also made accessory parts for other cars. This piece would be rather rare.