When there are unemployed resources in an economy, a fiscal stimulus does not crowd out anything. To the extent that it increases aggregate demand (and is not offset by tax increases, say), it employs resources that would otherwise be idle. It therefore crowds IN economic activity, and has no burden on the economy. The first thing we teach students is opportunity cost. What is the opportunity cost of fiscal stimulus? To the extent that it employs otherwise idle resources, its cost is NOTHING.
The canonical model is not relevant in this situation. The model has no explanation for an economic depression, and hence can not be expected to predict results accurately in a situation of persistently depressed economic conditions. It is only relevant when the economy is in a full-employment equilibrium situation. That is not the case now. It is for times like this that Keynesian economics was developed.
Why do our students hate economics? Because they come to us expecting to learn about the real economy, not some fantasy economy that exists only in economists' heads.