As a licensed amateur radio operator you can modify any radio for the purpose of using it in the ham bands
PROVIDING spurious emissions are kept at or below an acceptable level.
You can build your own gear from scratch, or hack an existing device for HAM BAND use.
There are even transmitters and receivers that a non licensed person can build, but the transmitter cannot exceed a certain power level and cannot transmit more than 500 feet.
Where a person runs into legal issues is that many licensed and non licensed individuals want to use the ham gear outside of the bands, which to a certain extent can be done providing you follow power restrictions.
But who actually check the power output.
Non licensed users can be tracked down by the monitors and often face stiff fines.
Licensed abusers face stiff fine and loss of license.
This information is easily found in any of the tech study guides and rule books, not just fcc guides.
But in the shtf scenario is anyone going to worry about fcc rules?