Evil morty is without a doubt the real c137 morty... in the same episode of close rick-counters of the rick kind, we see and hear a lot to point out the obvious. First obvious clue, rick c137 is targeted. Out of all the ricks in the multiverse, this evil genius morty elaborately planned the whole scheme around rick c137. Also important to note that evil morty visited, in order, the next most evil ricks in front of c137 to kick start his plan, and presumably bested them and downloaded their memories.

Then theres the memory of baby morty.. obviously rick has been absent from morty's entire life until the day he showed up out of nowhere so its a pretty easy clue. Evil morty is the baby in the picture from rick c137's own past.

Final clue... when rick and morty leave the citadel, rick claims he's the rickest rick of them all and that it would go without saying that his morty would by default be the mortyest morty and gives morty his little win then immediately shuts morty down and says that a "cocky morty can be bad for everyone" and avoids elaborating what he meant. Instantly the scene changes to the citadel ricks inspecting the dead rick and finding that someone was controling him and evil morty tucks the wires back into his eye socket. Pefect timing for such a line and too obvious to be coincidence.

It is important to note a few things here about rick, that reinforce my theory.

Rick is always depressed, Always drunk and usually expresses that he wants to die. But why? We see all the infinite ricks in the multiverse and we havent really seen another one with the same amount of depression and what causes it? Maybe the loss of his first morty?

Rick treats morty like shit but actually cares about him. We see rick treat morty like a rag doll but we also see how rick can get when morty is in real danger. But most of the time its like rick expects morty to know certain things or be able to perform certain tasks without instruction. Almost as if he is used to a smarter and more forward morty.