Manipulate soap request xml inside OnWriteStartBody(XmlDictionaryWriter writer)

Question

I'm calling a soap web service that is very strict on the request from inside an asp.net core API. by overriding OnWriteStartBody,OnWriteBodyContents,OnWriteStartEnvelope methods in the System.ServiceModel.Channels.Message class. my XML

    xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
    xmlns:ns1="http://POWebservice/">
    <soapenv:Body>
        <ns1:myaction
            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
            xmlns:xsd="http://www.w3.org/2001/XMLSchema">
            <trantype>purchase</trantype>
            <tranamount>94999.23</tranamount>
        </ns1:myaction>
    </soapenv:Body>
</soapenv:Envelope> 

what they expect :

    xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/"
    xmlns:ns1="http://POWebservice/">
    <soapenv:Body>
        <ns1:myaction>
            <trantype>purchase</trantype>
            <tranamount>94999.23</tranamount>
        </ns1:myaction>
    </soapenv:Body>
</soapenv:Envelope> 

i want to get rid of xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" I have been able to modify some of the payload using

protected override void OnWriteStartEnvelope(XmlDictionaryWriter writer)
        {
            writer.WriteStartElement("soapenv", "Envelope", "http://schemas.xmlsoap.org/soap/envelope/");
}

i have also attempted to implement the IClientMessageInspector but the intercepted message in BeforeSendRequest had only the header and not the body. plus it had too many hacks .