You obviously can't use KM to do this, since once the KM Engine quits it cannot be used to start itself.
To restart the KM Engine automatically, you will need to use a script:
property ptyScriptName : "Restart KM Engine"
property ptyScriptVer : "1.0"
property ptyScriptDate : "2019-12-29 06:43"
property ptyScriptAuthor : "JMichaelTX"
tell application "Keyboard Maestro Engine" to reload
Paste this into the Script Editor, compile, and save to the AppleScript Scripts Folder:
You can then run this from the AppleScript Scripts Menu.
If you want to assign a hot key to this script, you can use FastScripts .