Modify NSEvent to send a different key than the one that was pressed

I'm trying to create an OS X keyboard hook for assistive technology purposes (i.e. don't worry, not a keylogger).

When a user presses a key, I want to prevent the real keypress and send a fake keypress (character of my choosing) instead.

I have the following code:

- (void) hookTheKeyboard {
    CGEventMask keyboardMask = CGEventMaskBit(kCGEventKeyDown);
    id eventHandler = [NSEvent addGlobalMonitorForEventsMatchingMask:keyboardMask handler:^(NSEvent *keyboardEvent) {
        NSLog(@"keyDown: %c", [[keyboardEvent characters] characterAtIndex:0]);
        //Want to: Stop the keyboard input
        //Want to: Send another key input instead
    }];
}

Any help accomplishing either of those goals? Basically modifying the NSEvent "keyboardEvent" to send a different character. Thanks.