I have not seen the actual insides of the original but you can see thru the holes in the back in this pic from the original...
It's hard to tell how much space you've got, about two 9v batteries would be nice. Looking at the pictures it seems... enough for at least a few coincells, 6 leds, 6 resistors and a Mini, Pro Mini, Micro or Nano.
I guess you will also need a piezo sounder. These are small and flat, so won't take up much space at all.http://www.ebay.co.uk/itm/1-2-3-4-6-8-x-AA-AAA-23A-9V-Battery-Holder-Snap-On-Connector-Enclosed-Box-Switch-/180814661762?pt=UK_BOI_Electrical_Components_Supplies_ET&var=&hash=item2a1964c882
// set the LED with the ledState of the variable: digitalWrite(ledPin1, ledState); digitalWrite(ledPin2, -ledState);
Yes I will help. But I will not do all the work for you.
For the 4 LED's of the X in series. one could put them in series, and have a pin connected to the center point between the 2 pairs. then change the pin from input (light the pair of LED's connected to the 5V and then change the pin to an output (light the LED pair connected to the ground. that would save a pin on the '85.
would need to have the other two LEDs on a shared pin, using analogout to fade-up the leds.one pin for the sound and one for the on/off switch... or do we still have room on the '85