Depending how strict the author adheres to Freud's theory, and also what disposition is under discussion here, you can use 'drive-based':
The disposition has now transformed into a drive-based, sexual constitution.
This is a more general use of the word 'driftmatig'

Usually this is the Dutch word that refers to Freud's libido (which has a wider meaning than just sexual drive. It's a psycho-sexual energy, hence drive). So, in stricter Freudian use, an alternative translation would be:
driftmatig = libidinal
The disposition has transformed into a libidinal, sexual constitution.

The latter would be my preference, btw, because drive-based often just means "Freud's drive-based theory" - zijn op de driften gebaseerde theorie. It doesn't quite catch 'driftmatig' the way libinal does.

Rather than the correct word per se, getting the correct meaning of the text across is what's most important.

As you can see below, one formal effort to create a multi-language dictionary didn't get very far:
http://www.psychoanalytischwoordenboek.nl/termen.asp?nav=men...

http://dictionary.reference.com/browse/libidinal?&path=/
http://en.wikipedia.org/wiki/Libido
http://en.wikipedia.org/wiki/Drive_theory
http://www.terrapsych.com/freud.html