Yes, you have it too.
So, use either of these two aliases:
alias base64 return $regsubex($encode($1,m),/(.)(==?)/,$gettok(A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c d e f g h i j k l m n o p q r s t u v w x y z 0 1 2 3 4 5 6 7 8 9 + /,$calc((($findtokcs(A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c d e f g h i j k l m n o p q r s t u v w x y z 0 1 2 3 4 5 6 7 8 9 + /,\1,1,32) - 1) // (2 ^ (2 ^ $len(\2)))) + 1),32))
alias base64 return $regsubex(b64,$regsubex(b2,$1,/(.)/g,$base($asc(\t),10,2,8)),/([01]{1 $+ $chr(44) $+ 6})/g,$gettok(A B C D E F G H I J K L M N O P Q R S T U V W X Y Z a b c d e f g h i j k l m n o p q r s t u v w x y z 0 1 2 3 4 5 6 7 8 9 + /,$calc($base(\t,2,10) + 1),32))
The first continues using $encode and the "//" limits to versions equal or superior 7.41.
It's worth noting that $encode() is a left-to-right operation, while $base() is a right-to-left operation.