Popular Subaru engine overview
On the Sube engine front, the EA81, EJ20, EJ22, EG33, EZ30 and some versions of the EJ25 have had mainly good success in aircraft when properly cooled and with proper ignition timing and fueling.
The EA81 is an old pushrod design, suited for about 85-90 hp. Simple, easy to work on, robust, cheap to overhaul.
The EJ22 is a 4 valve, SOHC design, suited to make around 125 hp. More complex, reliable, plentiful still today. A turbo version is one of the strongest Subarus made with a closed deck block, tougher pistons and piston oil squirters.
The EJ25 comes in both SOHC and DOHC versions, again 4 valves/ cylinder. Many versions have a deserved reputation for eventual head gasket failures and some less widespread problems with valve guides moving down in the heads. A newer HG design seems to fix the first problem and the press fit on the guides can be checked during overhaul. The EJ255 and 257 engines are turbocharged models with some more robust parts and seem relatively trouble free outside of some isolated issues with the oil pump pickups coming loose with consequent engine destruction coming soon after. Atmo models are good for around 160hp, turbos good for 225 with low boost.
The EJ20 over in North America is mainly confined to turbocharged models. Has a reputation for being tough. Small displacement means you'd probably want to leave it turbocharged for aircraft use.
The EZ30 is a 6 cylinder engine, 4 valve, light, complex and compact, used extensively by Eggenfellner. Seems to work well, no widespread issues if fresh and treated well. Good for around 200hp if properly inducted and exhausted which the Egg conversions weren't.
EG33 is the older DOHC 4 valve six. Complicated but reliable. Good for 230hp and does not need to rev as high as the EZ engines to make power. A number of people are flying this engine successfully even though it was built in relatively low numbers compared to other Sube engines.
EZ36, which is based on the EZ30 has a bigger bore and much longer stroke. These generally have not fared well in aircraft use. Of the four I know flying, 2 suffered serious piston failures (one of them twice) and 1 had minor piston damage which was heading for major failure when taken apart to have forged pistons fitted per my recommendations. The stock pistons simply won't handle the continuous thermal loads imposed in this application. All these failures happened in under 60 hours of operation.
As with most auto engines, the stock pistons are usually the weakest link due to the much higher specific power output compared to traditional aircraft engines. Ignition timing needs to be kept short of causing detonation at all times and AFRs need to be kept rich (11 to 11.5) at high power to ensure reliability.
High time Subaru I've heard of flying in aircraft was an EJ22 with a reported 3800 flight hours (never opened in that time) in a gyro used for flight training in Australia. There are a few other EJs with right around 1000 hours and hundreds which have 400 to 700 hours on them.
You can pick up good used engines for $400-$1500 and rebuild them for $1000 to $3000 depending on parts used so the engines themselves can be a fraction of the cost of a Lycoming however you'll need some other bits like a PSRU, rad etc. to make it all work. Realistically a well overhauled four with a PSRU and all the bits needed can be done for $9K-$12K. Sixes $10K to $15K ready to turn a prop.
New EJ257 shortblocks are available for around $2K and a popular mod for aircraft use is to fit SOHC heads on them to save weight since high rpms are not required for aircraft use. This would be my choice for most RVs. I believe you could build a reliable 225hp from this basis for about $15K ready to run with a PSRU. Weight, depending on gearbox, would be around 10-20 pounds more than an angle valve IO-360.
We know the turbo EJ257s make good power as the one built by Ralph Inkster showed awesome speed and good fuel burn. Randy Crothers also built a 7A with this engine. Both would easily exceed Vne in level flight with modest manifold pressures at 8000+ MSL.