I’m not that familiar with Marklin or the couplers you describe but you can purchase Marklin coupler replacement sets on the Internet (I just checked)). The hard part is mounting them in the existing coupler pockets on your rolling stock, particularly locomotives.
One approach (for just rolling stock) is to get the so-called Talgo trucks with Marklin couplers (if they have them). A Talgo truck is a truck that has the coupler already mounted on it. These probably would be more expensive than just purchasing the coupler alone but it would possibly be a simpler conversion overall. You’d probably have to remove the existing coupler pocket from the original equipment however.

I should mention that some people don’t like Talgo trucks because they claim they are prone to derailments when backing a train up but I haven’t seen in with my N scale equipment which is almost exclusively Talgo trucks.

One final solution (though not as satisfactory) is to make up a ‘conversion’ car. This entails having a piece of rolling stock that have Marklin couplers on one end and the J type on the other. That way you can run both types of couplers but with obvious limits in flexibility. This suggestion works best with trains that usually are composed of the same cars all of the time such as passenger trains or long coal or ore ‘snakes’.

Hope this helps,

Geof