You'll still have a DC motor in them, gut the engine except for the motor, and run DC to the outer rails and center rail. You'll no longer have all the bells and whistles (literally) but your engine will run.
Some guy had actually done just that on a recent episode of I Love Toy Trains I saw on TV. He wasn't converting electronic engines though, they were all older conventional units.
All of my electronic stuff has been pretty reliable so far, so I can't complain. If it breaks I will probably go back with replacement electronics. Command control is a big part of the fun of it for me and also a big reason for getting back into the hobby a few years ago.