Yes, the problem was that the locos couldn't cope with slopes very well, much of extra power was needed uphill, and downhill it rushed. Also I wanted to be able to control the lights; especially the front light was very dim. As it showed later, mostly because the lamp wasn't correctly fit in it's hole, so the light wasn't spread in the correct direction.

So, the motor is a DC motor already, which makes speed regulation possible. The only thing needed was to get another decoder.

As I have an Intellibox as digital central unit, I'm not limited only to Märklin/Motorola decoders. The c90/c902 is rather expensive for my taste, and rather large too. And the speed steps aren't adjustable, which might be needed with this motor. So I started with the small Lenz LE130 decoder, which I had good experience of before.

Too my surprise there was too little space for this tiny decoder too! I was thinking about placing it visible in the drivers cabin, but it would have been ugly. So I measured the power needed for the motor, and as suspected with this weak motor, it wasn't too much.

Lenz LE040, intended for N scale locos, would suit very well with it's limit at 0.7 A or something, and dimensions smaller than the delta decoder, and two function outputs for front and rear light. A little more expensive than the LE130, but much less than Märklin.

The rest was straightforward. The hole for the lamp was drilled wider, the decoder was replaced, and lamps connected. Well nearly; when putting the shell back, the fit was so tight that one of the lamp cables was damaged and got into contact with ground. Fortunately this decoder has self test which could give me a hint, and some extra isolation could be applied.