Would it be better to repair the engine or just the time changing cassettes? on 2002 Ford Explorer

It happened all of a sudden one day after I got off work. I noticed a crack in my valve cover. I had it replaced and the mechanic told me that it was the time changing cassette. Now I have a collision shop telling me I need to repair the engine.

Asked by for the 2002 Ford Explorer
how many miles on engine?
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how many miles on engine?