Can you access an EC2 server from another AWS account

AWS publishes their public IP address space. You could probably try pinging some random IPs in those ranges and see if your packets get killed at the firewall.

If you build your entire infrastructure with CloudFormation you could more easily replicate it, even in another account.

You still need to deal with data migration, which will be different depending on your deployment setup.

If you use configuration management most if not all of your OS can be replicated in your other account too.

While AWS does have a method of reverse engineering a CloudFormation template based on your current environment I've never tried it, sounds like you should give it a go.

The aws cli has commands to move s3 data "aws s3 sync s3://from s3://to". You may need to export or otherwise share other data like RDS and SNS.