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"
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
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:
public class BuiltinFunctionsXsltExposer : IFunctionSource
Give your functions the same name as the builtin functions and it should work.
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:value-of select="Builtin:RenderComponentPresentation('tcm:5-267','tcm:5-261-32')" disable-output-escaping="yes" />
You will need to pass in valid TCMIDs of the component and template of course!