In 2003 coding you would do something like this:
IXMLDOMNode oXml = thisXDocument.DOM.selectSingleNode("...xpath to XML Node to be transformed...");
IXMLDOMNode oXsl = ... // You need to have your XSL loaded into an IXMLDOMDocument, and then select it's root.
string transformResults = oXml.transformNode(oXsl);
- - - - -
But the big question is this: Is all the rest of your code using the IP03 model, or the IP07 model? If IP03, then great, just get this part working in IP03. If the rest of your code is written in the IP07 model, then the right thing to do is to stop using "C# (InfoPath 2003 Compatible)" and move to using "C#".