Oh yeah... sagepay doesn't use a server behavior because it is all done with hidden form elements. That is why you won't see a server behavior for it.
You'll have to use a different technique in this case. Can you send me the success page? I'll take a look at it and find out what you can change to get it to open so you can work on the design without processing an order.
It might be easier to just create a $0.01 item and process an order to see the live result... but I'll show you how so you can decide which route you want to take.