Old coving repair

Thats easy,the side will give you a template...by hold a piece of hardboard,thin plywood up against it then draw round the coving.

Pin/nail another piece to the one that has the template then cut the template out. When done you will have to identical templates.

Measure the lenght of the bad section and add 2"(better bigger than smaller) nail cut another two lenghts of 1/4" plywood and nail or glue them to the templates sides so that these represent ceiling&wall flats of coving.

Get a good amount of plaster of paris and mix it well to you get gooey mix but not sloppy, pour it into the mold till it overuns the mold then leave it to set a bit,you will have to position the mold on the corner edge. When it set a bit enough to be still managable get a thin stick/rod and sweep it over the mold so that you are forming the contour of the mold, once stisfied leave it to set.

If you can get what I mean your a better man than I am. :-)

Or I'll get me coat.

I'd make a mould from glass fibre on an undamaged bit. Wax the section before applying the glass fibre. Made a pretty good job of repairing the stone mouldings on the outside of my house using this method and that was with mortar, so rather more difficult.

The original will be casting plaster.

Build it up with Plaster of Paris (or one-coat plaster) and a shaped scraper.