No they can't repair 90% of their tractor. The biggest issue is fault codes. If the software decides anything is out of bound, it throw an error which at best results in an annoying alarm and at worst shuts the tractor down completely. Even if the farmer can figure out what is wrong (which is not easy because Deere no longer publishes what the fault codes mean) and fix the problem, clearing many of these codes can only be done by a factory tech.Farmers can repair 90% of their tractor now, including parts. What they want access to by this legislation is the proprietary source code firmware. I'm not advocating for or against this, but can see both sides of the argument. After all, the farmer agreed to purchase the tractor with the proprietary code loaded on it. This code no doubt helps the farmers productivity (GPS navigation, site specific settings....) so they get a benefit from it (otherwise they would just use an Ox towed plow). The tractor companies put R&D money into their code development for this reason.
Think about Microsoft Windows OS. You can't have windows OS source code. But, if you want to "repair" your own source code, you're welcome to download and use Linux. Will you get the same productivity benefit from Linux windowsows? Maybe.
But this is all well beyond the philosophical scope of buying a baggie of orings for your regulator or an air filter for your truck...
If you want an accurate Windows analogy, it's not that they want to tinker with the source code. They just want to be able to restart after a system crash without paying Microsoft for a tech visit. Taking this further, they want to be able to install third party applications without paying Microsoft or go to the Services Manager and kill unwanted processes.