Hello,
Remember that since the versions of the ESXi 6.x product line you can no longer obtain a "free" product key, you can register and obtain it only for the currently supported versions 7/8.
For the rest, a VM is nothing more than a set of files usually placed in a folder whose name usually corresponds to the name of the VM itself so most of the time it is sufficient to copy those folders and their contents from one data store to another, just in general, at least I do that, don't carry snapshots with you consolidate them first and make sure the machine is turned off before copying it, not suspended.
Then I have to be honest, it's been several years since I had to manually copy VMs from one datastore to another or interact with ESXi 5.1 so you'll forgive me if I go from memory.
Regards,
Ferdinando