Can I replace the eccentric bottom bracket in my Salsa El Mariachi with standard BB? I have an older Salsa El Mariachi with a Bushnell eccentric bottom bracket with 54mm O.D.

Do I have any options to replace the EBB with a non-eccentric bottom bracket? 

The current bottom bracket has threaded openings to accept a standard 68mm English thread BB, currently running a standard GXP bottom bracket and crankset in the Bushnell EBB.
I've never seen something like that, an eccentric to concentric conversion BB. Even if it existed it would have all the issues of an eccentric, so the only benefit would be if you really wanted that otherwise unattainable chainstay length. If you really, really wanted this, just having one made would be pretty easy. If you're having creak troubles you could try a Niner eccentric, which has been known to solve the problem.