Quote:

Exploring the flywheel (Moroso?) having 130 teeth I found the steel surface (that contacts the friction disk) having a strange pattern. See pic.

With enough light it looks like cracks - but more than cracks these are edges from "smearing" the metal when friction disk rubs over it.

The clutch was not a problem before disassembling (I wanted to repair the transmission - I just took a look).

Is this normal for a used flywheel? Can i reuse it?



Those ARE cracks. Heat cracks. Stress cracks. Yes, you could reuse it as is, BUT you should not. It is normal wear and tear. Probably from some "spirited" driving and/or riding the clutch
I would not reuse it as it is. I would bet that anyone reading this would agree with me - re-surface the flywheel now. You are already this far into the project, what is another few days and a few more Euros?
The machine shop needs to cut (NOT grind) the surface of the flywheel to try and remove the cracks and the wear pattern area from the clutch disc (the smearing). It looks like they will at least remove 1/8th inch (3-4 mm). The cracks might be so deep that you will still see some of them after the resurfacing. This is a normal consideration when anyone replaces a clutch and pressure plate.
Speaking of presure plate, I bet the surface looks worn like the flywheel. You may consider replacing the clutch and pressure plate now. Or at least check to see if the pressure plate surface is not warped or worn (smearing). Use a staight edge and see if the pressure plate surface is flat. Some people have reused an old one by lightly sanding the contact surface.
At least replace the cluch disc (friction material).

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