If you're handy, it might be worth trying to take the keypad unit apart and seeing if you can resurrect it. If the whole keypad stopped working at once, I'd be inclined to think it's something simple like a connector that worked loose or a cracked common (ground) trace on the keypad PCB or maybe a door safety switch that's gummed up and not registering as being in the "safe" position.
Do be careful poking around the inside of the microwave if you check this. There are deadly high voltages present, possibly even when it's been off and unplugged. (The magnetron requires a couple thousand volts at a relatively high current, the better part of an amp. That's quite literally capable of killing the unwary.)
A
Panasonic .8 cubic foot inverter microwave oven would fit in the opening pretty nicely and most likely be an upgrade in quality from the original, too. The inverter microwaves perform much better than others when powered from a MSW inverter; you don't lose a good chunk of the cooking power, and the circuitry is somewhat more efficient as well, so you get more cooking from the batteries.