I built a VolkPlane VP-1 and we used a VW 1600 engine . These get direct prop coupling , as the prop speed is 2500/3000 rpm and the 4 stroke has the torque and low rpm power band perfect for prop.
But most Kit planes and Ultra lites us 2 stroke and that is where you will see the prop reduction set-ups. The 2 stroke have plenty of RPMs , but no power .. we call them Stop/Go engines High R s or None..
There is actually a kit conversion out that cuts the VW boxer in half and uses the 2 cylinders /opposed. It is preferred over the 2 strokes , which are usually Snow Mobile engines.
Mine had dual ignition , the standard 009 VW distributor as back-up,and a Wico Mag on the back of the crank.
For fuel, we used Posa throttle body/slide valve... some may remember them from the old Harley conversions.. Slide body metering, no floats/bowl.
The VW is actually ideal for aircraft as it is magneseum. opposed piston, air cooled and converts easily.. they use them a lot in small pylon racers on the West Coast..
The VP flew simular to the the old J-3 Cub tail dragger.. They had Continental engines , somewhat simular to the VW..
Days gone by.....