You might just bag it and replace the entire adapter. See: http://www.powerstream.com/ac-07505.htm

Anything 7.5 V and greater than or equal to 350 mA will work.

There is a little ICON on the adapter that can tell you if center is positive. You have to have the correct polarity.

The URL, I mentioned has an adapter set and a reverse polarity adapter available if you need it.

5.5/2.1 and 5.5/2.5 are vary common sizes. The problem is the 5.5/2.5 fits on the 5.5/2.1 receptacle.

With a caliper, you can measure the OD in mm. The ID can be measures using a known receptacle and/or a drill bit. For the 5.5/2.1 & 5.5/2.5, I use two of the known receptacles. If they both fit, it's the 5.5/2.1.

There are plenty of sizes to choose from: https://en.wikipedia.org/wiki/Coaxial_power_connector