Thank you all for your replies. I'd promised myself I'd learn how to use ngspice on Linux over the Christmas break, but FPGA seems to be the something I should have on the list. I do want to design a possible solution to the logic analyser, I certainly can't afford to buy one.
There have been a few comments about life span of memory technologies which are scary as hell! That makes me feel I should get my finger out.
There were a few comments stating that I should look at a wiring diagram for the tractor. I've written a blog post on that subject entitled "Power, Information and Logic" which deals with why a wiring diagram is useless, for certain information, in my opinion ;-). To give an example: from the wiring diagram I can see that the tractor's "Transmission Control Module" has, amongst other signals, a button for "Up" a gear ratio and a button for "Down" a gear ratio. I can also see from the wiring diagram that the tractor's transmission has 5 Clutches and 6 Solenoids controlling hydraulics in the transmission, as well as two potentiometers internal to the transmission which are inputs to the Transmission Control computer.
I can see all that in the wiring diagram, but if I wanted to control the various outputs to change from gear 13 to gear 14 what am I outputting to clutches and solenoids, and what am I expecting to see happen on the Pots? The wiring diagram is never going to tell me that. In addition to all that the Transmission Control Module has two 36 Pin connectors, not all of which are connected, I'll grant you, but there are over 60 connections to the computer. I've mentioned ones that make sense to me and are used in changing gears but are there other signals in the 60 odd which are also relevant? Is there an edge condition where an oil temp sensor shuts everything down or the gearbox will get destroyed? I've no idea, and even if I had a multi channel logic analyser it'd never pick up edge cases, but I might work out how to change from gear 13 to 14, most of the time.
Maybe I'll look at opening a computer module and see what processor and memory is being used that is probably the easiest way to future proof the machine, but I must learn about fpga. Having said that does FPGA do A/D? I must look into all of that...
Thanks for all your advice!