Hi snake42787,
Up front, I have no idea about a good replacement transmission. But several things could be causing the engine to have a "jumpie idle". Mostly common would be the crystalizing of the existing fuel. Unfortunately, this is a messy job to empty the tank and clean it out, replace the filters and possibly the fuel pump. You may want to run a good quality fuel cleaner (like Lucas) through the fuel system for a while to clean it out once it's been reassembled.
Another possibility could be the timing and/or points, cap and rotor, spark plugs and/or plug wires. I would suggest that all of these items be replaced/adjusted accordingly. Just because nothing changed on the vehicle since it was last driven doesn't mean that something didn't go bad during that time.
Hope this helps!