But realistically you would have to spend a lot to get to get the stock 60 hp to be more than 70 hp or so. To get 200 hp would cost thousands. The stock 60 hp of a dual port 1600 will get you up to 80 and cruise all day at 70 mph. You will get over 30 mpg, and it will last for over a decade, if you don't abuse it. Have you tried simply having a mechanic check your idle speed? It might just be set too low. It can fall out of adjustment over time. If so, the you might simply have a bad wire. But the throttle body is also likely. The PVC throws up oil and water foam, and it collects on the throttle body. The throttle body works by being heated and checking how cool the air has made it, so any insulator, like water and oil foam, totally screws it up. Take it off first before spraying it clean, so that you don't get carb cleaner solvents down the engine.
If nothing so far, then it could be the purge value or ERG, but let me know how it goes first. Open the air circuit from inlet to turbo and clean thoroughly. Particularly the air filter. Check the fuel circuit, particularly the fuel filter. Set both circuits and keep the idle at around 1000 rpm max. You already hit on one possible cause. Check all vacuum pipes for splits as well as all other hoses and connections, something is upsetting the balance. Listen to your friend about the vacuum leak. Just start replacing them 1 at a Time! Is the gear box in the Porsche 914 and the old Volkswagen Bug interchangeable? The 914 was a VW Type 4 engine. In the Type 4, it hung out the rear and in the 914, it was spun 180° so the mass of the engine was in the middle. I'll watch this discussion with interest.
Picture Taker is correct that the Porsche 914 transaxle is not at all like the old VW bug transaxle, even though they have the same bolt pattern and input shaft. Since the 914 was a mid engine car instead of the engine hanging out the back like a VW, the differential has to turn the opposite direction. The shift linkage also comes in from the side on a 914 instead of from the front like a VW. Even the output CV joint flanges are slightly larger on the 914, since it used the CV joints from the Van instead of the bug. That stuff would required major cutting and welding to mount a 914 drive train. Starting problems with VW 1600 dual port motor? I have a bug with a 1600 dual port motor (engine code is AH), and it cranks, but won't start. I've double checked all the electrical, made sure it gets fuel, and checked the spark, and everything seems right.
The only thing that doesn't seem quite right is that it cranks very slowly, even with a fully charged battery. Can this cause it to not start? And would the slow crank be the starter going out, or a different problem? There seems to be a problem if it is cranking slowly. It may be a starter motor issue and that is "sapping" power that should go to your HT circuit to fire the spark plugs. You could try a "slave" car/Battery and jump leads to see if that gets things going. 2 terminals together on the starter to see if that spins it fast enough. 3) You need to do a starter draw test. First make sure it is not binding, by turning it with a wrench on the alternator. Then put a voltmeter across the battery. It should read 12.5 at rest. Then try to crank it. It it drops below 10 or so, there is a problem. If when you stop cranking it comes back to 12.5, it is excessive starter draw.
You could try the starter bushing and clean the commutator. If it does not come back up to 12.5, it is the battery. If it does not drop below 11, it is a bad engine ground strap or dirty terminals. I always could just replace the battery and it would run, now it cranks but wont turn over. It is probably a fuel problem, but it could also be spark. To easily test, try spraying lots of starting fluid down the air intake. If it fires at all, then there is spark. It could just be the relay is bad from sitting. If it's been sitting for 2 years it may well be bad gas is the problem, stuff is highly volatile and evaporates over time. What you're left with in a lot of cases is varnish and deposits throughout the fuel system. If it's fuel injected you may need to pull the injectors and have them cleaned.If it's carburated you may have to get the carb rebuilt. If it fires up with the starting fluid you'll have your answer. If you can add some "fresh" fuel to the tank, as much as you can get in there to dilute the old gas that may help. I'd also check and make sure you're getting spark, and check ALL the wiring, It's not impossible some "critters" got up under the hood and decided to make a home for themselves.They like to chew on wiring. Check the entire air intake as well for nests.It's a favorite place for them to settle in to your engine.