You need to consider whether the lens was soaked enough for cat piss to get inside.
If not, any disinfecting household cleaner should do the job for the exterior parts. (Formula 409, etc.) Be careful and keep away from the glass and critical parts. Use lens cleaner on those areas just like you normally would.
If it has been soaked through, it's going to need to be taken apart to be cleaned. In that case, you need to decide whether the cost of having it cleaned is worth it. You might just be able to get a new lens from Ebay, etc., for cheaper than it costs to clean.

You could try to clean it yourself, provided you remember the old saying:
There are two kinds of people who would disassemble an antique pocket watch; watchmakers and fools.
Point being, that you consider the lens lost and the repair attempt to be a learning experiment. If you repair it, good for you! If you can't, nothing lost. You'd still have to buy a new lens.

Finally, I think you also need to consider what to do with the cat. If it is so old and so sick that it has to piss all over the house, maybe it's time for the cat to make a graceful exit from this life.