Hi Speedodude,
Its really tough to hear the things happened over here in Tamilnadu Workshop. As of my understanding, there is no perfect painter for painting old cars. Since it requires lot of paste work with perfection (As of my knowledge), Painters are missing in many places thoughout the bodylines..
Kindly dont for Isuzu gear box alteration until and unless your Engine things get solved. Surely this is not related to the gearbox. Something we need to check in the Engine area. Oil Pump, Engine Compression Power etc., So please dont spend too much money for Gear box thing without sorting this Engine Issues.
If you come to Coimbatore or Bangalore kindly let me know, we will meet up. In coimbatore there are some good places which we can do it with less cost too.. So dont worry too much..