Sounds like your clutch cable needs adjusting, or it might be stretched. Does not sound like an actual transmission issue. Have you tried adjusting it? What kind of shape is your clutch in?
Finding a 323 GT in a junkyard, or parts from one, is next to impossible. There were 10x or 15x as many XR2 made. Finding an XR2 transmission is not all that hard
You can order parts for the XR2 transmission, it's a Mazda Gtype and was used in many, many Mazdas/Fords/Kias with varying differences. I mentioned the 323 GT because it's the only other transmission that's identical and direct fit.