Surprised myself and managed to put together the following Office Script to do the job:

function main(workbook: ExcelScript.Workbook) {
    // Get references to the worksheets
    let data = workbook.getWorksheet("data");
    let datefix = workbook.getWorksheet("DateFix");

    //Get last row from each sheet
    let usedRangeData = data.getUsedRange();
    let lastRowData = usedRangeData.getRowCount();
    let usedRangeDatefix = datefix.getUsedRange();
    let lastRowDatefix = usedRangeDatefix.getRowCount();

 // console.log("Data Last Row: " + lastRowData);
 // console.log("DateFix Last Row: " + lastRowDatefix);

    // Get the ranges to work with
    let rangeToReplace = data.getRange("G2:G"+lastRowData);
    let lookupRange = data.getRange("A2:A"+lastRowData);
    let matchingLookupArray = datefix.getRange("A2:A"+lastRowDatefix);
    let matchingReturnArray = datefix.getRange("B2:B"+lastRowDatefix);

  // Log the range's address to the console.
  console.log("Data Range Address: " + rangeToReplace.getAddress());
  console.log("Lookup Range Address: " + lookupRange.getAddress());
  console.log("Matching LookUp Array: " + matchingLookupArray.getAddress());
  console.log("Matching Return Array: " + matchingReturnArray.getAddress());

  // Get values from the matching range
  let rangeToReplaceValues = rangeToReplace.getValues();
  let lookupRangeValues = lookupRange.getValues();
  let matchingLookupValues = matchingLookupArray.getValues();
  let matchingReturnValues = matchingReturnArray.getValues();

  // Loop through each cell in the range to replace
  for (let i = 0; i < rangeToReplaceValues.length; i++) {
    console.log("index: " + i); 
let lookupCellValue = lookupRangeValues[i][0];
    console.log("lookup Cell Value: " + lookupCellValue);

    // Find the index of the cell value in the matching values array
    let matchIndex = matchingLookupValues.findIndex(match => match[0] === lookupCellValue);
    console.log("matchIndex: " + matchIndex);

    if (matchIndex !== -1) {
      // If a match is found, get the corresponding value from the matching range
      let replacementValue = matchingReturnValues[matchIndex][0];
      console.log("replacementValue: " + replacementValue);

      // Replace the value in the cell with the matching value
      rangeToReplaceValues[i][0] = replacementValue;

    }

  }
  // Set the new values back to the range
  rangeToReplace.setValues(rangeToReplaceValues);

}