I would start by asking Kalkhoff customer service, then the German Pedelec forum https://www.pedelecforum.de/forum/index.php
Some German bike shops specialize in supplying older parts eg Fahrradprofis in Ravensburg can supply a first-generation Bosch Classic motor core. The used market is more mature in the EU eg https://www.tweedehandsfietsen.nl/tweedehands-elektrische-fiets so folk want to keep older model ebikes going.
I would start by asking Kalkhoff customer service, then the German Pedelec forum https://www.pedelecforum.de/forum/index.php
Some German bike shops specialize in supplying older parts eg Fahrradprofis in Ravensburg can supply a first-generation Bosch Classic motor core. The used market is more mature in the EU eg https://www.tweedehandsfietsen.nl/tweedehands-elektrische-fiets so folk want to keep older model ebikes going.
I do sympathize if you have already tried contacting Kalkhoff customer service and gotten nowhere, Kalkhoff had a rocky time with the Impulse motors, documented by frustrated owners on the UK Pedelecs forum, eventually Kalkhoff gave up on the design and switched to Bosch motors. It is likely there is a dwindling inventory of Impulse motor parts, being kept primarily to support in-warranty customers so its anybodys guess if they have parts for older motors. You are going to be relying on the used market so I suggested you post on the German pedelec forum in English but use Google Translate if you get replies in German, there will likely be current or former owners who might have information about any European sources of parts.
There's often no support for discontinued ebike electrical systems, some like Bionx direct drive motors can be salvaged if you open up the hub motor and remove the controller and gut the DRM electronics but then in the process you lose the torque sensor turning it into a throttle only motor that is illegal in the UK. Earlier this year Continental abruptly left the ebike mid-drive motor market, but put in place a stock of spares and a support staff to support the system while it remains under warranty for the next 3-5 years, Specialized did something similar when they stopped using GoSwiss motors and switched to Brose. Reputable companies like Bosch and Shimano will have a migration plan for ongoing support of older designs, but with others like Bafang the customer is out of luck if they decide at a whim to change the design, Bafang in the EU are trying to change however with the release of their BESST dealer diagnostic tool connecting their newer OEM motors to a small technical staff in the Netherlands. At some point you have to cut your losses.