I wonder if a wood powered tractor for farming would be more practical than a wood powered car for transportation

I think it would, the only problem being smaller row crop farmers who would be mostly likely benefit to implement it or want to implement it have been pushed out of agriculture more and more over the decades and struggle to survive at all. Which makes spending time and money on experimental work like this far less likely.

Check out http://www.driveonwood.com to see plenty of examples of both. A wood car or truck can be amazingly practical for any use involving long steady state (i.e. highway driving), not so much for city use.

A tractor can certainly work well on wood gas.