HELP! Isuzu LT134P no power to go uphill after changing gasket
 

Hi, I am a new member here. I have a bus model LT134P with engine model 6HK1TC. Recently, I have overheated the engine. But after repairing by changing the gasket, my vehicle have no power to go uphill. And I noticed my RPM meter is not working perfectly( going opposite direction).

The mechanics have already tried their best but they still could not find a solution.

Things i I have spent lots of money on repairs:
1) Fuel pump
2) turbocharger
3) checking of piston, con rod, and piston rings

i am very desperate to find a solution and hopefully any experts here can help me out with my situation.
Would greatly appreciate it.