That is a real nice find. perfect combo of pump, feedthrough, gauge, viewport, etc.
If your diffpump just has a KF40 on it, I would not bother hunting down a gate valve -- I don't think that tiny gate valves like that are even very common. I would probably just use a normal angle valve like this:
https://www.ebay.com/itm/NEW-Huntington ... Sw3yVZiVVH which will also have the side-benefit of making it less likely for an accident in your fusor to get dropped into the pump. My system uses a valve like this.
(this is more important for turbo-pumped systems while I think diff pumps are tolerant of having small metal bits dropped in them while operating, but still its easier to fish things out of the valve than out of the pump.)
These valves are extremely common, and frequently come up at a very good price on ebay. If you can drive a pneumatic valve those are much more common and cheaper as you have probably seen. Of course this would have the side effect that your fusor is now offset from your pump and slightly lower conductance, which I doubt matters. Personally I like the aluminum ones because light weight.