If I understood your question correctly, let's try to apply a payload template:

{
  "QueryLanguage": "JSONata", // Set explicitly; could be set and inherited from top-level
  "Type": "Task",
  "Resource": "arn:aws:states:::aws-sdk:s3:uploadPart",
  "Parameters": {
    "Bucket": "my-bucket",
    "Key.$": "$.key",                  // from your item/input
    "UploadId.$": "$.uploadId",
    "PartNumber": "{% $states.context.Map.Item.Index + 1 %}",  // JSONata
    "Body.$": "$.chunk"                // your transformed chunk
  }
}

Pay attention:
+ 1 in the payload template is required as Index is 0-based, but S3 requires 1-based indexing.

If you need more tips, look here:

• Transforming data with JSONata in Step Functions

• Simplifying developer experience with variables and JSONata in AWS Step Functions

• Step Functions - Map - ItemSelector with JSONata