I agree with dhouston.
Modifying the DS12A and MS18A will be a transmitting device and a FCC violation.
His passive radiators would not be any violation.
X10's transmitters are not overly well designed and soldering an antenna to the unit will probably detune the output.
If you want to see photos of their internal boards and schematics you can find them on the FCC web site.
Just use the FCC number on the back of the units.
Use the details choice for the device.
https://fjallfoss.fcc.gov/oetcf/eas/reports/GenericSearch.cfmModifying the SC1200As receiving antenna is a different case and should be no problem. Others here have done that.
Try mounting them on a spacer like maybe wood. To get then at least 1" to 2" away for the metal.