Well, ukmoose already explained how to ensure you extract the correct data from the json. Based on the picture you posted I presume that the properties you need to extract are: msg.payload_raw and msg.metadata.time.
I created a small flow to fake your data, extract the interesting information from the JSON data and decode the base64 to hex.
Good reference on how to decode base64 the simple way without using a contrib node (tks shrickus).`
If it works for you then it is missing only to handle the time information.
Flow:
[{"id":"d140ec3e.aed54","type":"tab","label":"Flow 3","disabled":false,"info":""},{"id":"180b562c.4ffb3a","type":"inject","z":"d140ec3e.aed54","name":"{ }","topic":"","payload":"{}","payloadType":"json","repeat":"","crontab":"","once":false,"onceDelay":0.1,"x":90,"y":180,"wires":[["196e91a1.312f8e"]]},{"id":"196e91a1.312f8e","type":"change","z":"d140ec3e.aed54","name":"Fake Dataset","rules":[{"t":"set","p":"payload_raw","pt":"msg","to":"/8BEhEP7Q7REQkUSQ5NEIUNDQspEZA==","tot":"str"},{"t":"set","p":"metadata.time","pt":"msg","to":"$now()","tot":"jsonata"}],"action":"","property":"","from":"","to":"","reg":false,"x":250,"y":180,"wires":[["5b344ff2.10fdd"]]},{"id":"5b344ff2.10fdd","type":"function","z":"d140ec3e.aed54","name":"Decode Base64","func":"let encoded = msg.payload_raw;\n//encoded = Buffer.from(msg.payload).toString(\"base64\");\n\nmsg.decoded = Buffer.from(encoded, \"base64\");\n\n\nreturn msg;","outputs":1,"noerr":0,"x":480,"y":180,"wires":[["723343de.fbb5bc"]]},{"id":"723343de.fbb5bc","type":"debug","z":"d140ec3e.aed54","name":"","active":true,"tosidebar":true,"console":false,"tostatus":false,"complete":"true","targetType":"full","x":650,"y":180,"wires":[]}]
