Identifiying and Cleaning leaked capacitor (which it was not)
Do the surrounding components give you any clues as to what function the capacitor had? If they're very large and are located right next to a transformer, it's probably a filter capacitor. Filter capacitors tend not to be too picky about their values, provided they can reduce the supply ripple to a level that the electronics can handle. If it's connected next to an IC, google the IC's part number and see if you can find an application note, or a usage example. Many datasheets have this information. This will show you what devices should be connected to it, and sometimes even their values, for normal operation. Sometimes this works, sometimes it doesn't. 

As for cleaning the goop, I would start by scraping as much of it off the board as possible using something like a popsicle stick or other wooden or rubber scraping tool (something soft enough to avoid scratching the soldermask and/or traces, but hard enough to remove the goop). Once you've removed the majority of it, try using 90% isopropyl alcohol and de-ionized (DI) water (careful not to let it get into any cracks that it would be difficult to get it out of). You can sometimes use other solvents, but use these at your own risk -- They may remove soldermask and even damage the traces. You can test its effects on a corner of the PCB if you want, where it won't harm anything if it does remove soldermask. Use a new toothbrush along with the isopropyl alcohol to help remove more of the goop. Finally you can rinse it with DI water and wipe it down with a lint-free cloth to soak up excess water and wipe off any residue. From there you can assess how much damage (if any) there is to the traces on the PCB, and determine whether or not it will need to be repaired.