[bombcar]

To expand on this, there are entire "fan clubs" around old military trucks - for example https://www.steelsoldiers.com - so you could infiltrate said groups and find what parts are starting to become rare/hard to find NOS anymore (new old/original stock) and start making replacements.

[primax]

There is a big fan club for Kubota kb250 tractors on Facebook with people making custom mods and implements. I'm sure there are lots of similar things out there

[ncmncm]

This is the way in. Garden tractor enthusiasts, VW camper van enshusiasts, vintage motorcycle enthusiasts -- they all want to customize, and will spend to get it.

A trivial electronic gadget to turn an oil cooler pump on and off is a good start. Electronic assembly isn't noisy or stinky. At most you might need to bend sheet metal for a case.

Guitar players love audio processing gadgets they can turn on and off with their toes. Make something that can be adjusted to give them a unique sound. Distortion is easy to layer on.