Firstly I want to offer you my condolences on your loss of Mooka. She was an adorable piggy and very much loved. Take comfort in knowing she is in a better place and is waiting to see you again someday.
As far as finding a companion for Midge, it is entirely up to you how old her next companion should be. Personally, as long as they were healthy and relatively friendly, I would adopt an older guinea pig as they have a harder time finding homes. Although you may not have them as long as you would a younger guinea pig, you would be giving them a forever home. One benefit of older pigs is that they are usually more calm and easier to handle as they have been around humans longer.
There are several different ways you could go about filling the void left by Mooka. One method (and personally, I like this one the best) is to adopt a M/F or F/F pair and introduce them to Midge. In the event that Midge or one of the other pigs were to pass, there would be two pigs left to comfort one another. Trios are becoming more popular in the guinea pig community for this very reason.
Or, if you don't feel like owning anymore guinea pigs after Midge, you could always foster a single pig/pair for a rescue and keep them in the same vicinity as Midge so that she knows there are other pigs in the room and won't feel lonely, but you aren't committed, so to speak. You could always foster up until she passes and then either foster the pig(s) you have until they are homed or return them to the rescue.
In the meantime, you could take something that Mooka slept or left her scent on in leave it in Midge's cage to help with the grieving process. Whichever decision you make as far as new guinea pigs, I'm sure it will work out in yours and Midge's favor. Again, you have my sympathies. RIP Mooka
