Try using a milwaukee keyhole saw that excepts sawsall blades. I use this all the time . Then I put a 24tooth hacksaw blade in and cut the lath. BNe sure to only cut away 1 full strip of lath and have the top and bottom land on the remaining partially cut pieces. Depending on the plaster you may or may not need to patch. Oh you will have to use a scewchissel to break out the plaster first. (screwchissel is an old scewdriver now used as a chisel, and sometimes can become a screwawl) This is not a slam bang operation it is a time consuming and you will need patience.
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