MyG37 - G37 coupe engine swapped won't rev past 2.5k rpm

G37 2010 engine swapped- won’t go over 2.5k RPM {HELP} [URGENT]

I bought a G37 2010 drove it for 10 days
The engine blew up.
I swapped the engine now the car won’t go over 2.5k rpm, the mechanic told us that it’s a timing issue on the new motor and you should get this changed for a new one ,
we did and the problem still won’t go away the code is get is P0021 which still suggests it’s the timing of the car,

present situation.
Car would run normally for 15 minutes , the engine light would come on and the car would go into limp again. won't go above 2,5k rpm
I have paid for labour twice and the cost of the motor has put me in a lot of trouble.

Another mechanic has told me it could be the intake pipe or the maf sensor as it has been damaged

anyone have ANY IDEA WHAT SHOULD BE THE FIRST STEP I TAKE NOW
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