Richard, that could be a 1300 gallon tank, While 1000, 1500, and 2000 are common sizes, perhaps someone had a 1300 gallon tank with two chambers...a 1000 gallon and a 300 pump station section. They will work for the purpose intended, and that may be the reasoning. The filled pipe could be because it was not a pump station at all, and was plugged that way.
Perhaps the pipe you think is filled is in fact the discharge filter. I'd do as Doug suggests: go get a 24" diameter riser pipe for whatever height.....put a cover on it and....done.
I'd avoid doing this just before or right after lunch, or you could think you are fishin' and become a 'Chum' bucket.
Having said that, I'd suggest you find out more about the existing system. Pump the tank, check the lines, find the 'D' box and camera the field. This IS like the FRAM commercial: Pay me now, or Pay Me Later. If it is in failure, you might as well know now.