On arranger jobs, I always have a 2nd keyboard on top for adding piano and/or bass parts in real time. I can always finish the job on that 2nd keyboard.
As far as amplification, I keep an old Peavy Minx in the car, and a Mixpad 9. If everything goes out, I put all inputs into the mixpad and that into the Peavy. If the job is big, I line out of the Peavy. If it is under 100 people, the Peavy does the job.
All associated back-up batteries, cords...even 3-prong adapters are in an emergency bag I have in the car.
Have had failures over the years, but have always been able to patch and limp through.
I would suppose that dining rooms-where I play the most, are easier to work with less equipment. In fact, I usually only add an arranger a couple of busy nights a week to add a little variety.
Whatever it takes...
Russ