The main absence is the entire cab glazing
Doesn't the Schoema use the same cab as the track cleaning loco? If so, the spare part list for 21671 on the LGB website . . .
. . . shows that its "Fenster" (E325055) is 15EUR and has a green traffic light for availability . . .
I'm not sure if they are willing to ship to Great Britain at present. If not, Modell-land is taking "requests" for a differently-numered version of the windows . . .
Set Fenster LGB 21670-M101 aus der Kategorie Reinigungslok 2067! Ohne Originalverpackung!Set Fenster 4-teilig für Schienenreinigungslok 21670
www.modell-land.de
. . . and if you mention that E325055 is equally good, they might be able to get that from Marklin for you. As I've written elsewhere, Modelland's postage to the UK is very reasonable for orders up to 20EUR.
Hope my sleuthing hasn't led you astray.
Best wishes
David