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What you need when doing a Six conversion:

An engine, of course. The best source here is Bruce Anderson's 911 book, as it covers all of the engines in detail. The real 914-6 used a '69 2.0 911T engine. Most conversions use a 2.7 ('74-'77) or a 3.0 (aka SC, '78-'83), as there are a lot of them, and they provide good power (160-180hp). SC engines are very reliable. 2.7 engines are cheap, as they have a bad rep due to poor design choices in handling emissions. The 3.2 engine is fairly common. The 3.6 is still pretty expensive, and pretty much maxes out the 914 gearbox. The older engines will usually need to be rebuilt, and this is not an inexpensive proposition ($7-12K). Get an engine with a complete induction system, as putting these together afterwards is difficult and expensive. The CIS (aka K-Jet) engines will fit, but only just. Carbs are often swapped in, but they're very expensive (Webers), or they're crap (early Solexes). The 3.2 and later engines all use EFI. The MFI engines (late 2.0 E and S, all 2.2, most 2.4) are very sweet, but MFI pump rebuilds are even more expensive than a set of 40IDA3 carbs.

For anything other than a 2.0, you'll need a flywheel. The 2.0 engines used the same cup-type flywheel with the push type clutch as the 914. The 2.2 to 2.7 engines can also use a 2.0 flywheel, as it will bolt on. The 3.0 and later engines can't use the early flywheel at all (9 bolts hold it on the crank instead of 6), so they need to be made or bought. Kennedy sells flywheels adapted to match the 914 gearbox to any Six.

An engine mount. The real 914-6 used a mount on the firewall with a single bolt holding the engine to this mount. Rich Johnson sells an updated version of this mount with two bolts, which is generally regarded as the best available. There are some bar-type mounts that attach to the 914-4 engine mounts, but these are of varying quality, mostly bad.

An oil tank. The 911 engine is a dry sump engine, meaning it has two oil pumps, one to suck the oil out of the tiny sump and into an external tank, and the other to take oil from that tank and pump in through the engine. The real thing had a tank that fit on the driver's side, forward of the rear wheel, aft of the firewall, between the inner and outer fender. Those indentations in that inner fender are cut out to make room for the oil filter, and the oil filler. Some people use a generic oil tank and mount it in the front trunk, sometimes also plumbing in a front-mounted oil cooler, too. Real 914-6 tanks, which were pressed steel, are hard to find. There are some knockoffs cast out of aluminum which frequently leak.

Plumbing. Real 914-6 lines are pretty much unobtainium, but they can be made up using custom fittings (available), AN fittings, and -12 to -10 hose. The exact fittings vary from engine to engine, but in most, there's a fitting under the engine mounted oil cooler that usually causes the most trouble, as you can't use the stock 911 fitting there, since the trailing arm is in the way. Cutting and fabricating is required here.

An oil cooler. This is optional for the milder engines (all T & E variations, SC is iffy). The 3.2 and later engines REQUIRE an external oil cooler, as they don't have one on board. The S versions of all engines will need an external cooler.

Exhaust. A set of 914-6 headers aren't too bad on price, but you get no heat. Stainless aftermarket HEs are a pretty penny. Used 914-6 HEs show up now and again, but are usually in pretty tatty condition, and still expensive.

Throttle linkage. The 911 uses rods for the throttle from the pedal all the way to the engine. Because the 914 installation flips the engine around, and uses a cable, some linkage is required to adapt the forward pull of the cable to a backwards pull to work with the 911 induction systems and linkage. A common way to do this is with a pivoting linkage back on the gearbox that uses a 911 throttle rod from the carb/efi linkage to back near the speedo drive on the gearbox, then run a cable from the pedal back to this pivoting linkage.

Engine tin. The 911 tin on the engine itself works fine, but the tin surrounding the engine one work at all in the 914 engine bay. Replica tin in fiberglass is available and quite inexpensive.

Depending on which mount you use, the shift linkage may need to be modified. The side-shifter long rod usually has to be cut and welded to straighten it, to keep it from hitting things (engine mount for cross-bars, HEs, the engine itself).

Note I have deliberately not included actual prices for things, as this information ages rapidly. A entire book could be written covering this subject in detail. I'm also only listing the things necessary to get an engine running in a car. Most conversions also upgrade the brakes, add flares for wider wheels, upgrade CV joints, etc.