If it allows Atoms, then most arm processors should be acceptable.
The IPC and architecture is obviously very important.
Even 1Ghz on one x86 != 1Ghz on a different x86 CPU.
And of course on a different ARM CPU.
Cortex-A53 is an ultra low power, tiny core. It's okay for an embedded toy, it's miserable for a real computer. You really really want at least a Cortex-A72.