You can get small expanding rubber plugs from your local auto parts store. They're sold in the Help aisle as emergency freeze plugs. That should do the intake and exhaust ports, and probably the water pump inlet/outlet too. Alternatively, you could just loop those together with a long hose.. I'd cap the PCV port with rubber stopper wedged in there.
The Alternator presents a bigger issue. Obviously, you can't leave it on there while you're blasting. It'll get crap all in it. I'd probably take it off and seal up the opening with some scrap sheet metal temporarily glued in with RTV or something. The starter hole can also be taken care of with an expanding plug.
Good luck. That crap gets everywhere.