Copy data from external List Item to New Form

Not sure I fully understand what are you after. ECTs are NOT stored physically in SharePoint, but rather queried in a lively-manner from the back-end system. You could however use CSOM ECMA Javascript to load from one particular item. Notice:

1. The actual ID is the BcsIdentity column - which is not a regular Integer but rather an ugly, long GUID like, e.g. __dg8100230003001300030013000300kc000d4001400e400k410084002500d20013008300
2. You absolutelly need to specify the columns to include both in the ViewFields and using the 'Include(BcsIdentity,Title) (here I'm only loading BcsIdentity and Title).

Use this in a CEWP that you could load somewhere to read behind-the-scenes the actual data from the Item you are targeting and than use it to perform whatever operations you are after! Example of Javascript based Query:

 var caml = '<View><Query><Where><Eq><FieldRef Name="BcsIdentity" /><Value Type="Text">' + curPageFirstId + '</Value></Eq></Where><OrderBy><FieldRef Name="BcsIdentity" Ascending="FALSE"/></OrderBy></Query>'+
                '<ViewFields>' +
                '<FieldRef Name="BcsIdentity"/><FieldRef Name="Title" Type="Text"/></ViewFields><RowLimit>1</RowLimit></View>';

            var ctx = new SP.ClientContext.get_current(); 
            var web = ctx.get_web(); 
            var list = web.get_lists().getByTitle("YourListHere");
            var query = new SP.CamlQuery();
query.set_viewXml(caml);
var listItems = list.getItems(query);
ctx.load(listItems, 'Include(BcsIdentity,Title)');
ctx.executeQueryAsync( 
function(){  if (listItems.get_count() > 0) {
  var listItemEnumerator = listItems.getEnumerator();
  listItemEnumerator.moveNext();
  DoSomethingElseHereWithEachRow(listItemEnumerator.get_current(), rowData, rowId);}}, 
function(sender, args) { alert('Request failed. ' + args.get_message() + '\n' + args.get_stackTrace());
});