I realize this is a really old thread, but it helped me find a solution to my similar problem and so I thought I'd add my 2 cents worth in case it helps somebody else.
I too had an old Rain Bird automatic sprinkler control valve (1 inch) where the valve itself had started leaking. I searched locally for either a 311A Irritrol or Champion PCL-100-C to replace the valve actuator with, but as earlier posters stated you can't find those at the typical Home Depot / Lowes type stores.
I had gone ahead and purchased the newer Rain Bird replacement (Model #DASASVF100 / 1 inch) but found out while trying to install it that the distance between the inlet and outlet pipes is much wider on the new style. This would've required reworking the plumbing of either/both the inlet and outlet pipes, which was not really something I was keen on doing.
Although I prefer Rain Bird over Orbit, I took a chance and purchased the Orbit Model #57624 / 1 inch sprinkler control valve. While the distance between the inlet and outlet pipes on the Orbit valve isn't a perfect fit to the Rain Bird being replaced, the difference was only about 1/8 inch and I was able to flex the pipes enough to fit the Orbit on without doing any extra cutting or splicing.
If you need a 3/4 inch valve instead, orbit has a Model #57623 which is also a closer fit than the newer Rain Bird replacement. I really don't understand why Rain Bird would make a replacement valve with different dimensions, but they did and now I have a hybrid system with Orbit valves and Rain Bird sprinklers. But at least I didn't have to rework the pipes any.