For a small one of < 50hp, not very difficult: a tractor is really just a very reliable engine (probably running well below spec), a transmission, a hydraulic p

For a small one of < 50hp, not very difficult: a tractor is really just a very reliable engine (probably running well below spec), a transmission, a hydraulic pump and a frame to mount everything onto. People DIY tractors all the time. For tractors the size that we're mostly concerned with in articles like this, no one who needs it would have the time!