As far as I know the 4*40 only comes as a kit, so this must be a plane that someone built and covered but never installed the engine and hardware? Assuming a kit, here are some answers..

There is no hatch for the fuel tank. You just slide it through the fuse opening. I do't know about relative widths but I used an 8 oz Sullivan slant type and an 8 oz Du-Bro is supposed to fit fine as well.

The kit comes with a cable-type throttle linkage but there is probably enough room to use a regular wire setup if you like.

The servos are mounted in a conventional manner so that you can see the top of the control arms facing you when you look into the fuse. You will have to glue in some hardwod supports and then buy a servo tray or make one out of lite ply. Try to keep it toward the rear of the bay if possible, but make sure the servos won't hit the wing or the aileron control arms when everything is installed. By the way the plans and instructions provide very little detail on this for some reason so everyone has to be a little creative.

I have an OS 46FX in mine and it came out very nose heavy. I ended up moving the battery pack to the fuse behind the wing and still had to add weight to the tail. My receiver is in front of the ply servo tray. I routed the antenna out of the fuse just behind the wing and stretched it out along the bottom.

I'll add a picture of my setup to my photo gallery this evening when I have time if someone doesn't post one for you by then.

Its a nice-flying plane and an easy low-wing to start with. If you over-power it like I did it will be very fast but lots of fun. Good luck!