Crown Vic motor replacement

So I bought my first 01gt a week ago. Turns out the replaced motor is NOT a 99 interceptor motor as the guy advertised. I’m keeping the car because I love it too much.

Is it worth replacing the heads on the engine and modifying everything up from there, or would it be easier less parts swapping to replace the motor entirely? I have a friend who is a master mechanic and I worked as a mechanic about 15 years ago so doing the work and the cost of labor is not the issue. In terms of what is a better way to go regarding overall performance and ease of installation. I would rather not ‘piece meal’ every little part.

Thank you in advance for any info or suggestions.
 
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