It's not that hard to do - a number of Australian guys built sizeable numbers of "home-built" tractors that were laughed at by the highly sophisticated Red, Green and Yellow tractor manufacturers - but these locally-built tractors got good support from their buyers - and their sales bit into the big brands sales.
These local tractor names are well-known to Australian farmers - Upton Tractors, Phillips Acremaster, Waltanna, Baldwin, Chamberlain.
Unfortunately, they all died out, due to either problems with sourcing components, pressure from the big American manufacturers, or from simply being bought up by the U.S. manufacturers.
There's a real need to simplify tractors today, and to get away from the ever-increasing requirement for factory technical specialists, and specialised equipment to repair them.
The Green tractor manufacturer has gained no friends amongst its end-users with their claims that their electronic technology is patented and cannot be worked on by independent repairers, changed, or replaced with aftermarket parts, without the Green tractor company effectively cutting off the tractor owner and leaving them stranded.
It will blow up in their face eventually, I don't think they have even started to understand the level of anger amongst owners who cannot do anything to their tractors, without manufacturer approval.
As one tractor owner stated, "I bought this tractor, I own it, I should be able to choose how I repair it, modify it, or do what I like with it, without facing manufacturer threats of lawsuits or withdrawal of support and parts supply".
The main problem with building your own tractor is ensuring continuity of supply of engines and drivetrain components.
With electrification increasing, this will lead to many manufacturers abandoning the manufacture of transmissions, and maybe even diesel engines, within a decade or two.