If you can get a small U bend of copper central heating pipe, 10-15mm,5/8" whatever your rubber pipe size is, you should be able to swap that in quickly without losing much coolant. Vice grips on the pipes will help. Yes you can put a basic water valve in bought from the same counter as the U bend. Crude but effective. The copper pipe should be held inside the heater box on a couple of tabs and that will mean that the pipe doesn't just pull out, unfortunately. Many of these have been "got at" so might be just stuffed in which may be lucky for you. Mine is just bent up and tucked up in the airflow, not in the box at all.
If you do get the Ranco valve from NAPA, here's a how to on a similar HCV.
http://www.pbase.com/czechman/ranco