Hello Hans,
For getting a better running you need to install a so called 6090-type (high performance) motor.
It needs a 6090-type decoder. Both parts come in kit form as 60901.
This will resolve the problem in digital mode and, very probably, will enhance the running behaviour in analog mode as well.
Installing a 5-pole anchor could be a partial solution but, take care: you can't use the anchor from a 6090 motor, you have to install that anchor used in 35xx Loks. Both 5-pole anchors are different in diameter and coils.
In regards to flickering lights, the light bulbs must be isolated from chassis and the return pole be connected to the orange wire from the decoder. This is the best way for eliminating the flickering.
Have fun!

Another option whould be to buy the 60901 motor parts separately. (for partnumbers see: http://www.bogobit.com/decoder/dectype.html#60901) These parts fit in almost any drum collector motor, so it's unlikely that will cause any trouble. To control the thing you could consider using the ESU 51604. This decoder is capable of switching between pick-up shoe. And yes, it's likely it will be capable of switching interior lighting. Front lights are not a problem at all.

There are several points here:
Motor: the 6090-type is the best option you can use.
Decoder: I don't like the way UHL76400 decoder handles the 6090 motor, I'd use a Lokpilot instead.
Pickup shoe: if your Delta currently supports the pickup shoe switching, leave it to control that and use a Lokpilot to control the motor and the head/backlights.
I don't see that Uhlenbrock device for switching the pickups in their page, ESU offers such a device for use with Loksound/Lokpilot.
A different approach: leave the motor as is and install an UHL76200 to control your Märklin-AC motor. It has load control over the CA-motor and 1A output for lights and functions. Leave the Delta for pickup switching.
You have several choices, it could be hard task to make a decision.
Best regards,

Thanks to all for your (varying) suggestions. Indeed, not easy to choose which option, but definitely enough ideas for some fun in the near future! (Btw: selling it is out of the question; it looks great next to the VT11.5. Things like outdated couplers don't bother me at all)

Greetings from Holland,
Hans

Hi everybody
Finally, I got the opportunity to tell someone about my conversion of my old 3071:

60904 (60901 in case of 3471) and a home made circuit board with four relays to control directional pick-up shoe change, internal light (2 x 610080 connected in series per car to reduce the light intensity and power consumption) and front/rear light. That way a flicker free light is possible without using the "orange +", which requires an extra wire through the train set and the current load to the decoder is drastically reduced.
White front lamps are 28 volt T1 bulbs with wire ends in M* gray plastic sockets, red light is original with a serial resistor.
f0 = front/rear light
f1 = internal light
f2 = driver's cabin light
f4 = brake delay disconnection

I also installed the interior from 39700, including the heavy steel plates between the interior and the car floor. I am still looking for people to put inside the cars, in particular the restaurant.
My next step will be to install an MFX decoder with diesel sound, but then I do not know how to make the pick-up shoe directional change.

You're right - I needed them for the 3071! (sorry for any possible confusion).