You have to open people.txt file in the game directory, and then find the entry for the Ferrari engine.
There are a couple of lines there that should be associated with the info "exclusive to Ferrari". If I remember correctly, either deleting those lines or replacing those lines with 0 should make the engine become available for all teams.
You can look at how the other engine entries are structured and deduce it from there.
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On a side note, congrats for the purchase - that is an awesome game if you're a fan of 1990s/ early 2000s F1!
Let me know how your career progresses and if you managed to fix that Ferrari engine thing!