Well it's the head and ECU for sure at the very minimum. However, also bear in mind that the 24V vans came with ESP as well and so the ECU is probably involved in expecting the inputs and outputs for that too, integrated with the braking system. So might need something more such as a different ABS controller? Unless the new ECU can be "told" via VAGCOM that there is no ESP system present and turned off.
Are you really sure it's going to be worth doing this though? If you can get the chance to drive a 2001+ Eurovan Camper (compare apples to apples weight-wise) with this engine, you might be surprised that the difference isn't as big as you think. I drove a later van and it didn't feel significantly better than my 12V 2000 VR6 at the time (and the owner of the other van drove mine). We each agreed that the drive wasn't that different. Both engines have nearly the same torque (177 ft-lbs on the 12V and 181 ft-lbs on the 24V, only 4 more). The only place where the 24V will outpace the 12V is at high revs with its higher redline and the ability to rev higher and breathe better. But you (I hope) don't drive all over winding it out to 5000 rpms.
Go for a drive in one if you can and see what you think but I bet you'll be underwhelmed at the difference.