I don't know of any Mitisubishi experienced auto repair shops in OC, but there are handful of garage mechanics around here. I have also found the SoCal Monteros forum on Facebook to be very helpful. I redid the engine on my 2003 3rd Gen about 2 years ago now, it still runs! Photos here: https://nwoods.smugmug.com/Vehicles/Montero/3rd-Monty-Restoration/
Update: A few years ago I did have some help from a company near Santa Ana called Swedish Asian. I think they've changed names, but same owners/mechanics and such. They are decent, fair, and not a bargain.