Here's some reading on it... https://www.thesamba.com/vw/forum/archive/index.php/o-t--t-114707--.html (https://www.thesamba.com/vw/forum/archive/index.php/o-t--t-114707--.html)
If it's a Hummel, it is most likely one in the list below.
2 CYLINDERS ENGINES
28 hp 88 mm bore x 69 mm stroke 840 cc 85 to 103 lbs.
Options – New Case , Battery ignition, Dual ignition, Bed, Radial or Mosler mounting, Starter, Alternator
32 hp 92 mm bore x 69 mm stroke 917 cc 81 to 103 lbs.
Options- New Case , Battery Ignition , Dual Ignition, Aluminum NiCom Cylinders, Bed, Radial or Mosler Mounting, Starter, Alternator
37 hp 92 mm bore x 78 mm stroke 1037 cc 81 to 103 lbs.
Options- New Case , Battery Ignition, Dual Ignition, Aluminum NiCom Cylinders, Bed, Radial or Mosler Mounting, Starter, Alternator
45 hp 94 mm bore x 86 mm stroke 1200cc 84 to 102 lbs.
Options - New case, Battery Ignition, Dual Ignition,Starter,Alternator Bed, Radial or Mosler Mounting. ( Note ) This engine comes standard with Aluminum NiCom cylinders.
All 2 CYLINDER EGNINES CAN BE MADE WITH THE CUT CASE OR FULL CASE VERSION
You could position the engine with a spark plug hole as the highest point and the piston on BDC before the exhaust stroke, then fill the cylinder with a light weight oil and measure the amount of oil it takes to fill the cylinder. Probably have to loosen the exhaust valve adjustment a little so it's completely seated to keep it from leaking