When possible, I prefer to keep and repair the original pieces, but sometimes they were so far gone that I chose to rebuild them from scratch. Here’s an example, a window crown on the West wall that was too far gone.
First, carefully remove the piece from the wall, taking note of details. Here, see that the builders had a plan for avoiding water penetration: the top plate has a dadoed slot and there’s a vertical strip that fits into it. In this case, someone has added a flashing on top.
It seems to work, there’s no sign of water damage on the sheathing
Once a piece is off the wall and on the workbench, the first step is documentation.
Draw all the pieces so you know it goes together. Measure all the pieces. Take profiles of any moulding.
Draw out construction details. Note any complications. Here, the moulding had to be custom made. There were enough entablatures to rebuild that it was worth the expense.
Much as I’d like to save the original brackets, that’s just too much 150-year-old crusty paint. Plus, it’s more fun to make new ones.