Conversion - 1/2 Zumwalt
How2 Convert To A 1/2 VW
Start with a 1600 VW beetle engine (1971 or so). Maybe just buy a rebuilt long block. Plan to convert it to 82mm stroke x 94mm dia pistons. This configuration should provide 40hp at about 3700rpm.
The 82mm stroke is chosen because this is the largest stroke that does not require the inside of the case to be trimmed for rod clearance. The 94mm piston size is the upper limit before cylinder and fins start getting very thin. The case and heads will need to be bored for the 94mm. Have this done at a machine shop (about $100).
A dual port head can be cut in half to make two heads. The rocker shaft can be cut and used with just one rocker stand if two small dowels are installed between the support bracket and the head stand to prevent any twisting. PCV valves can be used in each valve cover for the crankcase breather. Just cover the extra holes with 1/8 in aluminum plate.
Cylinders & Pistons
Use a 94mm big bore piston and cylinder kit that comes with rings to match.
Crank & Camshaft
The 94mm bore and a 82mm crank will provide 40hp at 3700rpm. A forged counterweighted stroker crank is best. Cut the crankshaft and camshaft. The #1 (thrust) main bearing should be moved to the #3 bearing position (by the cam gear) with one flange cut off next to the cam gear. Then the #3 bearing should be moved to the #2 bearing position with the extra flange next to the crank throw. Now you can sand the flange to get the .003 to .005 crank end play. Some folks balance the crank, however others try it as is and if it vibrates too much, they put a Balance Master on the prop.
The most popular distributor is the 009 distributor with a 3 ohm coil and a 5ah 12v battery for ignition. Ground the extra spark plug wires.
No oil cooler is needed. High volume oil pumps push to much oil so it is best to use the stock oil pump.
There are numerous options but one proven combination is the 32mm Mikuni carb with two 90 deg input pipes from the heads to a Tee. This puts the carb on top of the engine case. One 32mm carb on each head works also.