Those scopes look way too thick compared to the screenshot I linked, in the screen it has a small opening where you look into it and a wide exit but a thin tiny tubelike mid-section.. Those are all pretty much thick from front to back.. You sure its a zf4?
I am no expert, but it looks way more like a ZF41 when compared to googled up images. The ZF41 also has a rather large diameter in front and gets significantly thinner in the back.
What one may assume that is in the game are ZF41 for MKB42 and G41. The sniper version of the k-98 may have different scopes too, but I would not know which.
In the end, the problem is not the availability of sights, but whether they could be mounted. MP40 and PPsh do not seem to have any way of mounting any different sight than they have as a standard. There simply was no picatinny rail system back then like it is the craze now. And consider: With the MP40 and PPsh you would not shoot precisely at longer ranges than 50-100 meters max. The standard iron-sights would be completely satisfactory for this cheaply and mass produced weapons.
As said, there is line of individual upgrades for every weapon, but no true customization as in other games.