Once you accumulate hundreds of portal keys it seems the only way to see if you have a particular key is to endlessly scroll through them all. Is there a way to do a search or at least sort them?
Unfortunately the inventory doesn't have a lot of options to sort the items. Once you've selected the "Portal Keys" voice from the menu, you have the option to sort them by Distance or by Title. You can also tap on the scrollbar under the keys to jump to that spot in the list, so if you approximately know where the key should be you have a minimum advantage, but that's all.
At the moment these are the only options to search for the keys, so my best advice is to memorize their approximate position, either by sorting them by title or distance, whichever works for you. By distance usually is the easier way, since usually you interact with portals that are closer to you.
In case you need a quick reaction (i.e. a fast recharge while being attacked) you can always tap on the portal name in the log to open the Remote View; if you have the key of the portal you'll have all the options available from the keys in the inventory.