There is no template, but just use the revenant.
There is no revenant template, but I don't think you should worry about it. Unless you let your players read your monster stat blocks during combat, they will likely not notice the difference between the old and new stats for this enemy if you use the revenant stat block as-is and just convince them it's the orc war chief through the narrative. You'll know the difference, but your players aren't psychic, even if we as DM's are often paranoid that they are.
If you describe the revenant's appearance as a recognizably dead and rotted version of the former orc that discarded its weapons to mete out vengeance with its blood-encrusted fists, you'll get player buy-in without having to make any changes to the revenant stat block whatsoever. In my experience, having used a similar stat block swap on many occasions, the most that players tend to notice is a difference in AC, and but usually only if they happen to also have DM experience. Even if they make such a comment on a difference in AC or other properties, your players are likely familiar with how easy it is to hit zombies and other undead, much easier than their living counterparts in general, and with how mindless and instinct-driven they can be. It's an easy cover story.
If you want the revenant to have a bit more orc flavor, you can add the orc war chief's Battle Cry feature, but only if it has minions and only if it was iconic enough in a previous battle for the players to remember its use. By contrast, unless you were to announce "the orc uses Gruumsh's Fury," that feature would be indistinguishable from a higher damage bonus, and unless you were to announce "the orc uses a bonus action to move," the Aggressive feature is indistinguishable from the enemy simply having a high speed. You wouldn't gain anything but complexity by adding those two features to the revenant, and the revenant's features are already useful and flavorful.
So... the simplest solution is to just use the revenant as-is and narrate how you want the players to perceive it. Less prep, less fiddliness, less chance for confusion at the table.