A 25 year old tractor design (probably out to 75+ year old tractors even) still has hundreds of millions of dollars in infrastructure and tooling behind it. Molds or dies for a single part can easily reach into 6 figures, without taking into account the $10 million injection molding machine.
The simplifications you could do would be limited, as every design decision affects the placement and access to a dozen other parts.
Take a part like this, which is for a Deere 750 (small tractor) released in 1989: https://www.compactractorparts.com/tractor-parts/john-deere/...
I could model that in CAD in under an hour. Lining up manufacturing is a minimum 12 month process with preexisting relationships, and would take upwards of $250k to get in the door. This also doesn't take into account the "societal collapse" angle - post-apocalypse there's no way to make that yourself, and manufacturers in China would presumably no longer be an option.