Migrate Site from aegir Drupal hosting platform to Acquia hosting
Disclaimer: I do not use Acquia and don't know their service.


  Should I need to move the code as a normal multisite setup to Acquia?


The documentation seems to say that multisite is supported: https://docs.acquia.com/cloud/multi-site 

You would probably need to copy the platform from /var/aegir/platforms. If you look in that directory, each directory is a pretty normal Drupal multisite codebase.


  Since the codebase is extracted from aegir environment, I don't have any files on the drupal root like install.php, update.php?


The platform is just a normal Drupal codebase, it should have these files (even if it doesn't use them).

The hard part will be to move the databases, you will (presumably) need to create them in the Acquia frontend, and change the settings.php file of each site to use the new database info. Aegir creates a drushrc.php but in a non-Aegir environment you can just take the settings and move them into settings.php.

Hope this helps.