A transfer case to make your tractor 4WD?
If so, this isn't a practical project. Better off finding a used 4WD tractor.
However, if you're determined to do this...think belt drives (gears and shafts would be mighty expensive). You'll have to determine the correct drive ratios to ensure the front and rear wheels turn at the same speed. A Machinery Handbook ($60 or so if you buy a new one...pretty cheap if you can find one in a used book store...the math and gear tables haven't changed in about 80 years, so used should be fine) would be a good starting place to determine what the ratios should be.
I hope you're good at math, 'cause you're going to have to do some before you fabricate anything.
You're also going to have to figure a way to drive half shafts (front axle) so you can steer.
If the transfer case is for running another application (PTO or hydraulic pump), the Machinery Handbook would still come in handy.