Posted this in the other thread, but I'll duplicate it here. Enjoy my delicious copypasta!
A few things to think about-
There are a handful of 1:25 ag tires I've seen on diecast Ertl promos, but I've seen just as many (if not more) 1:1 car/tractors with medium or heavy duty truck tires on the back. I'd also look into WW2 era 1:35 and 1:24 military kits- some of those might work, and many 1:35 tires will fit bi-scale wheels and still look 'right' on the model. As Dr. Cranky mentioned, the reissued Willys Jeep kit's tires would be suitable as well. You could even fabricate your own spoked steel wheels, like the kind seen on the Model T conversion kits sold by Shaw. Also, some retained all four stock wheels/tires, or had a set of builder-fabricated dual rear wheels. Sometimes the stock wheels were cut down to fit a different diamter wheel rim.
Ertl did make a few 1:25 farm and construction kits, but the drive tires would be WAY too big for what we're talking about here- the tires in those kits are meant for a 14 to 20,000 pound machine, not a flyweight like a doodlebug.
You could use pretty much any early '30's or older vehicle as a starting point, though the vast majority were Ford based.
Model T Doodlebugs normally had improved cooling, as the Model T engine wasn't as well-suited to field work as it was to simply moving the car. Often the frames were reinforced, and in many cases the rear axle was solid-mounted to the frame- no rear suspension at all.
Also keep in mind that no two will be exactly alike, even if they were based on an aftermarket kit like the Shaw setup. Builders equipped them and set them up for their own uses.