I need to buy 2 new 31" Geolander MT Tires, my current spare is only a very old 30" can I use one of my old directional Geolander MT as my new spare, even if I had to run them in the wrong direction for temp use? Or should I just buy a cheap 31" spare?

As long as you don't have a locking diff, it should not be a problem. It may take a little work to make the spare fit in the stock location though.

If you do I would run it in the Front and stay out of 4wd. That would mean moving a tire back if you have a flat in the rear but better safe than sorry.