Please try the stylesheet below:
<?xml version="1.0"?>
<xsl:stylesheet version="1.0"
xmlns:def="http://schemas.microsoft.com/sqlserver/reporting/2008/01/reportdefinition"
xmlns:rd="http://schemas.microsoft.com/SQLServer/reporting/reportdesigner"
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
exclude-result-prefixes="def rd">
<xsl:strip-space elements="*"/>
<xsl:output method="xml" indent="yes" omit-xml-declaration="yes" />
<xsl:template match="/">
<xsl:apply-templates select="def:Report/def:DataSets/def:DataSet/def:Fields/def:Field[@Name != def:DataField]"/>
</xsl:template>
<xsl:template match="def:Field">
<Field>
<xsl:copy-of select="@*"/>
<DataField><xsl:value-of select="def:DataField"/></DataField>
<xsl:copy-of select="rd:TypeName"/>
</Field>
</xsl:template>
</xsl:stylesheet>