Hi Gamolina
First, do not attempt to do it yourself unless:

• you have an illuminated magnifying glass
  
• you have a set of miniature tools for clockwork repair
  
• you are extremely skilled with your hands for miniature / minute works

I checked on the Märklin website and IMO they never published an exploded view of this crane (really too bad)

All the points below are what I suspect.
I have never opened my crane (it works, why would I?) so I am just trying to help but without any responsibility on my part
The first thing to do is to take plenty of pictures (to help reassemble the crane later)
The second thing is to remove all wires for the hook and the boom having noted exactly its path in the various pulleys
The third logical thing to do would be to separate the crane body from its wagon base:
Here is a picture of the top of the crane


In the red oval, you may see 2 screws serving the purpose of separating the crane from the wagon


Next to remove the casing screws under the crane body
Here is a picture of the underneath of my Goliath 49950


Take plenty of crisp pictures during the whole process.

WARNINGS
- be prepared for springs to jump and parts to fall down. Probably the Märklin repair team has special tools to repair.
- Märklin does not provide spares except for their own shop ; Whatever you find, you will have to fix it yourself.
- avoid force rotating motorized parts (rope drums, crane body) unless absolutely forced to.

Sorry for all this grim warnings but that is reality
Cheers
Jean

Edited by user 06 October 2020 21:28:54(UTC)  | Reason: Not specified