Contact Caterpillar.
They have units up to 6.7MW; I'm sure that they can reconfigure one of their gaseous fueled units to run on propane instead of natural gas or one of the new biofuels.
Your biggest problem will likely be keeping enough propane on hand to keep the unit running...these things are very thirsty.
Be sure to size it so that it can handle starting currents of your larger motors and elevators.
Remember that if this is going to be your sole power source for more than the duration of a minor outage you will need to use the rating for 'prime power' instead of 'standby' or 'emergency'.
