If you can't find a 'real' tacho cover - in the past I've done 2 things to fix guages like this. IF the size happens to be a 'fit' you can get a CD case (you can usually buy blank ones for like $! each or less at a computer/electronics shop.. Mark out the diameter you need then cut it - obviously it will snap with scissors so the trick is to use a hot knife of some sort (I actually have a soldering iron I use only for plastic cutting or welding, but you can also use your 'regular' one if you clean all the plastic off the tip after using it and re 'tin' the tip so it will work as a regular soldering iron again. The tip can be used to make the holes the screws go through too.

Cut the new piece a bit larger than needed then carefully sand down the outside edge till it is down to the right size. put masking tape over it to file/sand flat the area around the screw holes so the new 'window' will be able to be tightened down for a close seal/fit.

I've done this a number of times and it works very well. It takes a little bit of time to do it (maybe an hour start to finish) but comes with a certain sense of satisfaction. If there are lots of micro scratches on it, a cotton wool ball and some 'brasso' brass polish (or whatever brand of brass polish they make/sell in your neck of the woods can polish it to a fantastic result too (incidentally that also works very well to reclaim CDs or DVDs that have tiny scratches that cause read errors - works as well as any higher priced 'special' cd polisher).