I can't see what base64 has to do with this 🤔

Hex2Ascii (Compose)
Add more values from Ascii table
{
"21": "!",
"30": "0",
"31": "1",
"41": "A",
"42": "B",
"61": "a",
"62": "b",
"63": "c"
}
Select
From
chunk('6162633031', 2)
Map
outputs('Hex2Ascii')[item()]
Compose
join(body('Select'), '')
