It is possible to assign a scriptblock to a variable

$scriptBlock = { ... }

Also you have an error in your code: You check the connection for $2FQDN (instead of $1FQDN) in your first if.

Additionally, you could simplify your code further: both bodies of the if-else are identical.

My suggestion:

$scriptBlock = {
    # Loads of registry keys...
}
$faultReport = @()
$computerName = $null
if (Test-Connection $1FQDN -Quiet -count 2) {
    $computerName = $1FQDN
}
elseif (Test-Connection $2FQDN -Quiet -count 2) {
    $computerName = $2FQDN
}
if ($computerName) {
    try {      
         Invoke-Command -ComputerName $computerName -ScriptBlock $scriptBlock
    }
    Catch {
        $faultReport += $computerName
    }
}

Note: Always use proper indentation. I can never stress that enough. It makes reading and troubleshooting your own code so much easier.