Sage pay integration with power store is not a simple process, you have to add all of the pieces to the checkout process manually. The documentation we have for using Sage pay (ProTX) with eCart, has you run the checkout wizard to create the pages, the the checkout pages are already created for eCart, you cannot run the checkout wizard to have them created, you need to manually add the sage pay behaviors to the page and manually create the checkout notify page.
One thing you could try doing is to run the checkout wizard in another site to see how the behaviors are applied to get a feel for how the PowerStore pages will need to be modified.