The VW Type I/Type III engine that all these conversions are based on maxed out at 1600 cubic centimeters, and 60 SAE gross horsepower, as they came out of the factory. This was developed at a propeller unfriendly 3000 rpm. They were intended for automotive use, no thought was given to having one power an airplane. If you're at all familiar with these engines, they have a very restrictive intake and exhaust, and the power made per cubic inch is low. This makes them ripe for hot rodding, and racers get a surprising amount of power out of them, at a cost of vastly decreased longevity and reliability.
Most of the experience I have with VW engines is with friends who raced Formula Vee. In that application, they use a nearly stock intake and get maybe 45 hp from 1200 ccs. Not much output, but adequate for an 800 lb race car. They're reliable in that application, but the top ends get rebuilt frequently because like any other engine, they lose a little power once the valves are worn in, and in racing every bit counts.
@kyleb mentioned R. A. Hoover's writings, and I think Hoover is correct. All engines are cooling limited, and that is usually manifested in the cylinder heads. if you look at the last air cooled Porsche racing cars, they had liquid cooled heads, as Porsche couldn't get competitive levels of power with air cooled heads. Rotax has the same setup in the 912 series, air cooled cylinders, liquid cooled heads. As it came from the factory, the VW engine made around 52 net horsepower at its peak, so Hoover's contention that you can reliably get 60 horsepower and no more sounds reasonable. That should be enough for a light, clean single seat airplane. It would be helpful if that 60 horsepower came in at a more propeller friendly engine speed, with the VW spinning as fast as it does at maximum power, the prop has to be small, and I think that thrust would suffer somewhat, particularly during takeoff and initial climb.
The Aerovee folks claim 80 hp at 3400 rpm Great Plains claims 80 at 3600 rpm. Both builders have increased the displacement of their engines to over two liters, but IMO haven't dealt with the limited amount of cooling that can be done with the stock heads. What is propeller efficiency at that speed? I can't think it's good. How much thrust will you have for the takeoff run? You're going to have to run a very small prop on either of those engines. I also think you're not going to want to try to get 80 hp out of a VW Type 1 engine for very long, I'd be very concerned about the cylinder head temperature while burning that much gasoline.
If it were me, I'd give the Revmaster engine a look. Joe Horvath has designed some custom cylinder heads that are installed on that engine, and they've held up well in the racing world. It also makes its rated 82 hp at a more reasonable 2950 rpm. If you're going to be doing aerobatics, you're going to be climbing a lot, and the cylinder head temperatures are going to reflect that.
I'd still keep my eye on the cylinder head temperatures.
