Hello Sir,
I have experience working on AWS Infra for more than 5 years.
There will be two scenarios here
1. Infra is already deployed and need to added under cloudformaton
2. Infra will be deployed as per cloud formation script.
For Point no. 1
It will be difficult - to get infra under cloud formation,
I will suggest to have a staging Env. with cloud formation script but with striped of hardware ( Instance type)
once we add all the components take the script to other region and create infra from that cloud formation script, this will help to check if all the components are covered under cloud formation and also that site can be use as passive DR site. ( for dry run)
Point no. 2
This will be relatively easy depending upon size and components (complexity)
This is how my approach will be to analyse the Infra.
However you have not given any details about how the current Infra is deployed, I would like to discuss with you the same over chat if you are ok with it.