A 512K trapdoor expansion is hardly worth repairing; it's probably much easier to get another one.
Otherwise it's seems cleaning up battery mess isn't enough. You'd have to fix any damaged traces. A yellow screen does indicate a RAM problem (an exception takes place before the OS setup has progressed far enough to e.g. display a Guru) - I assume, you've got a 512 K chip and 512 K 'fast' config, so the OS data is put into expansion RAM - which isn't working correctly and crashes the machine.
