Repairs to broken stained glass pieces is much more difficult than new creations.
Disassembling pieces is a challenge. I'm sure that professionals have tools and techniques that make my efforts seem childish, but I use a hot soldering iron to melt the solder seams. The molten metal drips and splatters onto the workbench, and with effort & time I remove enough to get the pieces apart. If this sounds easy - it's not! I use the broken piece(s) and what is left intact of the rest to create a template for the replacement piece. I use those to cut the carefully selected replacement glass and re-assemble the whole.
Disassembling pieces is a challenge. I'm sure that professionals have tools and techniques that make my efforts seem childish, but I use a hot soldering iron to melt the solder seams. The molten metal drips and splatters onto the workbench, and with effort & time I remove enough to get the pieces apart. If this sounds easy - it's not! I use the broken piece(s) and what is left intact of the rest to create a template for the replacement piece. I use those to cut the carefully selected replacement glass and re-assemble the whole.