You take a big end mill and you start milling (cutting) untill the thread depth is correct. Zenoah does this on the cylinder for the G-26 EI. The G-26 magneto uses the CMR7H, so they face it down and install the CM-6 in the EI version.
WE DO NOT OFFER THIS SERVICE. Unless you know sombody who works at a machine shop and can do this on the side after you take the engine apart. You might be better converting it to EI and run just gas.