What would be the post-apocalyptic engine of choice? Something that would have to be durable, and built to broad specifications- not many precision tools/lathes in this future.
So what would be a practical design for Mad-Max type vehicles (buggies and motorcycles)? And what would you power it with? I don’t know how simple it is to refine gasoline, especially with crude resources/equipment/knowlege. Maybe having something that runs staight off crude oil? Biodeisel? Pee? 
My vote would go to the two-stroke diesel. A relatively simple engine that could be slightly modified to burn biodiesel, kerosene, diesel, jet fuel, etc.
Wow really? If that is true, than that WOULD be really versatile! Maybe add a timing switch or something so you could tweak it to run on different fuels…Hey, you never know what you’ll find out in the Australian wasteland…
Actually jet fuel, kerosene and diesel are very similar to each other, so much so that you could probably burn kerosene in your diesel engine without too much harm. A two-stroke design gives you some simplicity and more power per weight. Not needing spark plugs will be nice, since you may not be able to get replacements easily.
All that being said, you will need some precision equipment to make any engine, be it diesel or steam, or whatever.
But you will need a working DVD/videotape player for what you find in the Australian wasteland.
Ummm…steam? Not joking: a steam-powered car used to hold the land speed record. Assuming this is a post apocalyptic society, there may be quite a few machine parts to scavenge, but finding, extracting, refining and maintaining any petroleum-based fuel source is going to be a problem. Water, on the other hand {assuming you’re not in the Outback} is free.
Of course, then there’s the problem of roads to run it on - hopefully they’ve survived Armageddon, since I wouldn’t want to take a heavy steam-powered vehicle, carrying water and fuel, off-road.
Which brings us back to petroleum-based power, which lightens the load, but you’re still going to need some kind of pneumatic tyre, and I’m not sure about the rubber supply, let alone manufacturing and maintenance.
Let’s face it, mankind has spent all but the last 150 to 100 years {if not even more recently} with the horse as the primary means of motive power. Giddyap!
A Sterling-cycle engine can get by with only about 4 moving parts, and because it’s external-combustion it can run on any source of heat: coal, firewood, solar collector, etc.
It’s probably at least as easy to build as any internal-combustion engine. But I doubt you’re going to have much luck given the tools and materials you’d find in the Australian outback.
I thought two-stroke diesels needed a supercharger? Doesn’t that add some manufacturing obstacles?
Well, I drive a Nissan Xterra that runs on gasoline. But that isn’t what you’re looking for here. Post-Nuke, I might go with a small honda motor out of their ATV’s. My neighbor has one of their 450’s and while he is always replacing handle bars, rims, tires, armor, etc. from wrecking/racing, the motor seems well nigh indestructable. Given that there are literally millions of honda’s around the world from two strokes up to 4 bangers there is a huge pool from which to vampire parts.
A bicycle.
Simple to build…cheap to run.