Taken together, the MBTA is staring down a staggering budget deficit in the fiscal year that starts next July, according to an analysis by the Massachusetts Taxpayers Foundation. That’s assuming fare revenue rebounds to 75 percent of its previous levels. While some bus routes have recovered more than half their ridership, others aren’t close to that level. Most subway lines are closer to 20 percent of their previous ridership, and commuter rail trains are especially empty. It’s a grim forecast that seems to put service cuts on the table, said Andrew Bagley, the watchdog group’s senior vice president of policy and research.