I don't think there is an alternate replacement for ACS in NAC. A basic NAC environment consists of three components: a Network Access Device (NAD), an authentication, authorization, and accounting (AAA) server, and a posture agent running on a NAC-compliant host. NADs are often Cisco routers or Cisco switches, the AAA server is the Cisco Secure Access Control Server (ACS), and the posture agent is the Cisco Trust Agent (CTA).