How to remove the transmission of a GS20

[Franck French]

Unfortunately, my boat has lost its transmission (no forward or reverse traction—it feels strange when you're water skiing…). The shipyard (there's no Super Air Nautique specialist in my area) says the entire engine needs to be removed to replace it. They claim it's due to how the engine is mounted to the hull. That surprises me! However, I haven't found any video or manual explaining the steps to repair the transmission.
Can anyone help me find resources to understand how to proceed?

My ship : GS20, 2017, PCM ZR409.
Thank you so much.

Franck