I agree with you but I have a new thought. The P.O. removed the factory air box and installed a cone filter directly on the turbo. The car appears to have some type of MAF assembly in front of the factory air box. I'm assuming this would throw the entire AF mixture off if it's not having air drawn through the MAF? Just a thought since I know the 80's Mitsubishi heavily rely on the MAF to run correctly.I cannot tell you what to do but normally throwing parts at a problem does not fix the problem.
If you would like to diagnose the problem let me know.