Hello,
I like to summarize here some conversions I did (or will do) on my Märklin BR 221 from the starter kit 29811 (similar to 29720).
The head light issueSeveral people are unsatisfied with the gloomy head lights and white is shining through the rear light section.
Therefore, I tried to apply LED lights for red and white color and to find a better position to separate white from red. To tell the truth: There is a good and a bad message!
Let's start witn the good messageFor the head light, I used a warm-white LED from a LED stripe, soldered short cables to use the original socket. I was able to postion the LED in such way, that it shines directly into the fiber glas but not to somewhere else. A 1kOhm resistor reduced the voltage and current.

As You can see, the flare light was reduced effectively to a minimum and the head light shines bright and clear:

For comparison, this is the standard light with bulbs:
The bad news: Something happend, maybe a short circuit, I don't know. But the AUX amplifier of one side is dead now. Therefore, I will install a function decoder for the lights, when I create the final solution for both sides.
Ball Bearings I thought the driving performance of this loco wasn't so bad! But during a discussion I was thinking about, if I had already installed ball bearings to this loco or not. And NO, I didn't! Therefore, I made a short check on my desk and I realized, there is place for improvement.
I mounted a ball bearing into the motor shield:
The second ball bearing fits into the gear box:

Now, the motor sound is much more quiet, the loco starts driving smoothly in the 1st driving step. The current is between 200mA and 300mA without load but with maximum speed.
Here is a video of the loco with motor sound switched on and at the end again with motor sound switched off
The maximum speed is 140km/h (meassured with
meassure waggon), which is exactly the model speed!
Silent pickup shoeThe standard Märklin pickup shoe 206370 can be replaced by the Märklin silent pickup shoe E138079.