When the MGA (or early MGB) gearbox dipstick gets loose in the housing it may be at risk of falling out and being lost. The prior article noted that the felt piece is available as a replacement part (at least in the UK). But you might otherwise find a suitable rubber stopper that can be used as a repair piece.

On October 26, 2013, Del Rawlins in Anchorage, Alaska has this suggestion.
"Looks like I have found a solution for replacing the grommet on the gearbox dipstick. I found myself at the local industrial hardware store. They had a small drawer full of "marine plugs" and the grommet on the 3/4" version looked like it would be about right, once the flange was cut off, and it looked like the hole through the center would be smaller than 1/4".

Here is the plug disassembled. I had to remove the rivet in the cam lever to get it apart. All we are interested in is the rubber piece.

I carefully cut off the flange with a new single edge razor blade, taking care to cut as square as possible. Then assembled it on the dipstick, in place of the felt.

It seems to work good. It's a bit of a snug fit in the hole in the gearbox case, but that's kind of the idea. The hole in my gearbox only measures 0.710" roughly, so it takes a bit of force to push it down there. I found a possible source of these plugs online, if anybody else is interested. If the plug ends up being a little shorter than what you need, you could always stack another washer or two, or trim the flange differently for more length. Mine was a perfect fit just cutting that part off entirely.

I put some Parker Lube (O-Ring Lubricant) on it, and it now goes in and out of the case easier.