think of it like this. 4 strokes to the engine are starting at top dead center, 1)piston goes down, intake valves open, suck in air and fuel, 2) piston goes up, compression stroke then fire from plug 3) piston goes down, power stroke 4) piston goes up exhaust valves open and pushes exhaust out, then back to top dead center and stroke 1 again.
Now backwards 4) exhaust stroke, 3) power stroke, 2) compression stroke, and 1) intake stroke then back to 4 again
With the exhaust stroke coming immediately after the intake stroke, that would empty the cylinder of fuel leave nothing to combust and burn in strokes 3 and 2.
I honestly dont see how it would work unless the crank could turn backwards and the valve train would still be going forward. If that happened it would be the same and workable. Not saying it cant be done, it would be tough. Plus the point about the oil pump pumping backwards would have to be addressed also.