What do you think of an air cooled Volkswagen engine for building a light two-seat airplane?
This is probably the only engine I can get my hands on that needs no reduction gear and has enough HP. What I look for in my airplane is to get me in the air at a not-so-high altitude, speed is not a must as long as it can climb and get me high in the air, I would stick to 80miles/hour, not looking to be a competitive pilot.
The resemblance between the air-cooled engines sold in Volkswagens for more than three decades and a conventional light aircraft engine is no coincidence -- Volkswagen chose this engine type because it was lightweight, reliable, avoided some of the problems endemic to liquid cooled engines of the late 1930s (when the original Volkswagen design was drawn), and in the day fuel efficiency (specific fuel consumption) and emissions were very much secondary considerations (if they were even looked at).
The biggest difference between a VW engine and one of the smaller Continental or Lycoming flat-four air cooled aircraft engines is dual magneto ignition. This redundant ignition (the engine will run on either magneto and spark set) is a critical item for reliability, and is not present in the engines as made by Volkswagen. The conversion therefore requires new cylinder heads to accommodate dual spark plugs, as well as adding dual magnetos to supply energy for the spark plugs. Battery and coil ignition, as original to Volkswagens, is not preferred for aircraft, as it's very desirable that the engine continue to run even if the entire electrical system has failed for some reason and the battery is completely flat.
The other common conversions are aircraft type updraft carburetion, carburetor heat, and sometimes still a belt or gear reduction drive (these engines typically deliver best power around 3500 rpm, while light plane size propellers are usually happiest below 3000 -- the engine can be run slower, but will be more efficient if allowed to rev a little higher).
Beetle engines have been successfully used in small homebuilt planes for many years. However, they are not ready-to-use in aircraft: they require a number of modifications to successfully convert them from car to airplane use.
Those conversions are performed by companies that do this as a business and then sell the resulting VW aircraft engine as a finished product. You can do the conversion yourself if you have a properly-equipped machine shop and a good set of plans.
Get in touch with the EAA organization https://www.eaa.org/eaa which organizes home aircraft builders. It may literally save your life.
Experience in aircrafts engines are gained at high risk. You do not want the engine to break down and stop when flying. There has been numerous car engine conversions done and some of them has worked, some not. Best is to get in touch with people that has actual experience and can help you avoid doing the same mistakes that has already been done and show you solutions that work. Even very small details may be important when running the engine at 80% max power for long durations. In comparison, in a car the engine often runs at about 20% max power.
You might elect to select to check on using a conversion kit from a company that sells these. It will save you on creating the experience and cost might not be too large. A quick search shows as example this page: experimental aircraft info
-- addition Seems like the magazine Kitplanes is still around. Check their website and you are bound to find a lot of links to help you. And again, remember that flying can be safe but it can kill.
Your selection criteria are "any random, car engine available to me". There's a problem with that. You're looking at peak car engine power and mistaking it for continuous engine power.
Drag racers have built 300 kg Chevy V8 engines that output 1500 kW... for 9 seconds. Contrast with
The fact is, most cars cruise at maybe 25% of their maximum power on old-tech engines like the VW... less than 10% today now that a V-6 sedan has a "250 kW" engine in it. (Sedans take less energy to cruise than they ever did, because of weight and aero drag improvements, yet the engines get bigger and bigger max power ratings for competitive reasons, and because EFI (FADEC) has just gotten that good).
The result is that car engines are not designed to be able to cool at continuous 60 - 100% power, as is expected out of an aircraft engine. If you attempt to run a car engine that hard, you will get untold thermal problems from oil coking to cylinder warping, because the engines were just not conceived to run that hot continuously, and so were never designed nor tested for that.
As such, you would have to dramatically de-rate the automotive engine to keep it at a power level it could sustain continuously. At that point, you would have a very heavy engine for the power you are getting.
