When I run into situations with quotes inside strings I use QUOTENAME.
In your case I would do like this
DECLARE @item As varchar(100) = 'IsNull(Modifier_3, {0})';
SET @item = REPLACE(@item, '{0}', QUOTENAME('', ''''));
INSERT INTO Export..DataDictionary
VALUES (..., @item, ...)
You can of course make it a one-liner if you like
REPLACE('IsNull(Modifier_3, {0})', '{0}', QUOTENAME('', ''''))