You will need to invoke the (globally available) function RenderComponentPresentation as explained here.
The builtin functions are found in Tridion.ContentManager.Templating.Expression.BuiltInFunctions but I have no idea if SDL attributed them with an XSLT namespace...
Assuming they did and that namespace was "http://www.sdltridion.com/ContentManager/BuiltinFunctions"
you would declare an XML namespace on your stylesheet like
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:Builtin="http://www.sdltridion.com/ContentManager/BuiltinFunctions">
and call the RenderComponentPresentation builtin function via
<xsl:value-of select="Builtin:RenderComponentPresentation($componentId,$templateId)" />
If they did not you will have to create your own assembly, put it in a TBB; load the TBB into the XSLT via
<?XsltMediator extensionsTbbId="tcm:193-34-2048"?>
at the top of your XSLT TBB.
Your assembly would just be passthrough functions for the builtin functions but the class would have the XSLT namespace attribute wired up as below:
[XsltExtension(Namespace="http://www.sdltridion.com/ContentManager/BuiltinFunctions")]
public class BuiltinFunctionsXsltExposer : IFunctionSource
Give your functions the same name as the builtin functions and it should work.
Update:
Thanks to Rick who has confirmed that builtin functions are available in XSLT if you use the right namespace declaration the following code should be correct:
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:Builtin="http://www.sdltridion.com/ContentManager/FunctionSource/BuiltInFunctions">
<xsl:template match="/">
<xsl:value-of select="Builtin:RenderComponentPresentation('tcm:5-267','tcm:5-261-32')" disable-output-escaping="yes" />
</xsl:template>
</xsl:stylesheet>
You will need to pass in valid TCMIDs of the component and template of course!