I have a forklift with a commercial slant 6

Which parts have you replaced when you say you've "replaced the electronic ignition system"?

I don't think much of your "two seasoned heavy equipment mechanics" -- it sounds to me like they're taking random guesses and throwing random parts at the problem rather than doing proper diagnosis. You've got fuel (how do you know/where and how are you checking?) and spark (how do you know/where and how are you checking) when the engine will run...which one of them do you NOT have when it won't run (how do you know/where and how are you checking)?

It's not valves too tight -- if they're tight enough to make the engine fail to start, it would barely run (if at all) ever.

This problem is not going to be difficult to fix, but it's going to take proper diagnosis.