We are rehabbing a 120+ YO house and trying to stay within the restoration covenants of Preservation NC. Aside from the standard cracks, door/window gaps in the plaster (we know how to fix these) there are large, irregular shaped sections of plaster missing on many walls where the lath is still intact behind. Some of these are a foot or more across at the widest. Are there any products on the market that make repairing large sections of missing plaster easier than doing it the old-fashoned way with lath & key? Preservation NC is OK with a modern solution as long as it "looks" like plaster.

You could try fast drying joint compound, setting compound. You can get it in 20minute or 45 minute bags but it isn't much easier than real plaster. Are you planning this as a DIY project?

You could try filling in these areas with dyrwall. Use a comparable thickness of drywall to what the thickness of the plaster is, use a piece slightly larger than the hole, trace the outer edge of the drywall onto the plaster, cut out that excess plaster, and fit in the drywall. It will be a perfect fit every time, which will save a lot of work. Then, all you'll need to do is to tape and finish any gaps and sligtly fill in any lower areas with joint compound. Sand and paint.

You could try fast drying joint compound, setting compound. You can get it in 20minute or 45 minute bags but it isn't much easier than real plaster. Are you planning this as a DIY project??

Thanks! Since I posted I have done some research and found that Sheetrock 20 or 90 do a good job and are a little easier to use than plaster. Are these the products you are referring to?

You could try filling in these areas with dyrwall. Use a comparable thickness of drywall to what the thickness of the plaster is, use a piece slightly larger than the hole, trace the outer edge of the drywall onto the plaster, cut out that excess plaster, and fit in the drywall. It will be a perfect fit everytime, which will save alot of work. Then, all you'll need to do is to tape and finish any gaps and sligtly fill in any lower areas with joint compound. Sand and paint.

Thanks for replying! I have been thinking about using this process but was concerned about trying to nail the pieces to the lath in between studs - lath would tend to flex a lot and pull away from existing plaster. Would gluing these to the lath prior to taping and finishing do the trick?

Yes, good idea to not use drywall since it will not attach to the lath strip as well as plaster of drywall compound in the key. Nail drywall to the lath will likely break off more keyes and make your job tougher. You found the right products but there are some between 20 and 90 if you want to consider them.

Thanks for replying! I have been thinking about using this process but was concerned about trying to nail the pieces to the lath in between studs - lath would tend to flex alot and pull away from existing plaster. Would gluing these to the lath prior to taping and finishing do the trick?

You should always glue drywall to whatever you are attaching it to.
You can screw the drywall fast to the lathe with no problems.