Help finding engine builder in or around Stuttgart

Probably best to bring it to a KTM dealer.
Near Stuttgart there are several: Kosak, Sarholz, Bauerschmidt

ADAC is an international series. There's the national German championship too. Depends on his level I guess, there are many regional series as well. Around Stuttgart the closest one is probably BW Cup:
https://www.motorsport-wuerttemberg.de/meisterschaften/motorrad/motocro…

If your friend wants his confidence to be crushed he should try ADAC MX Masters. I got state champion in Germany in 2019 and wanted to try the MX Masters. The first guy ( Kevin Strijbos ) was around 15 seconds a lap faster than me.
After that I thought I should just stick to the regional races. Good times.

Yes, if KTM, Kosak or Sarholtz

We raced in Germany for the last 10 years, in the ADAC Supermoto Series, there are lots of tuning houses, some good, some not so good, but the beauty of it being Germany is that you can use tuners in Holland, Belgium or Italy as well..

As has been said, the regional series will be fast enough for 90% of riders to not win in,

https://www.dmsb.de/de/motorradsport/motocross


The DMSB is the organising body for the races and the ADAC promotes the bigger races , so DMSB is like the AMA, and ADAC is FELD , and they promote pretty much all motorsports in Germany.

The tracks will be decent, and there will will be local run club tracks , nearest big track is Reutlingen which is a GP quality track south of Stuttgart.

I will see what i can find out, we used guys in Belgium and Holland for our 4 and 2 stroke motor work, but ended up using Eric Gorr for our CR500 .

I reckon he just needs to get out there and ride, and stick a pipe and a map in the 250.

Apparently they're is a place behind a strip mall that would do a bang up nikasil job in the 80's..... I've heard it a million times on Pulp interview commercials