There isn't anything official; FinCEN doesn't publish much in the way of "how to." Simply put, when you buy another bank (or branches of another bank) you buy their BSA program; if the branches have CIP problems or unreported suspicious activity then the surviving bank has those problems.
A part of your due diligence, prior to the acquisition, would be to do your own BSA examination of however much of the institution you are buying. In your specific case, you might gauge your level of concern by considering the fact that regulators rarely visit branches.
Regardless, homogenize their BSA/AML efforts ASAP.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.