Hoses, fittings, fuel and oil lines, and the sizes and thread forms that have to match before any of it seals.
Dash sizes, tube diameters and thread sizes for 37° AN/JIC fittings, with what each size is normally used for on a car.
Standalone engine management: what the inputs and outputs actually are, how to wire them, and what the settings mean.
Sensors, ignition, fuelling and the mechanical side of making one run.
Engine and gearbox specifications for the B, D, H/F and K series — the part-by-part numbers the calculators are preloaded with.
Or read everything in the wiki, newest first. Every page says where its numbers came from, and every page can be corrected — they live in files you can send a change to.