It sounds as if the position of the mounting holes for the keystart controls the center distance between the flywheel gear and the starter gear (which is plastic). If that is right, the center distance would need to be correct within a few thousandths of an inch (unless you wanted to use up a lot of starter gears). It sounds more practical to somehow find a base engine that already has the holes in it (it may not need to be in good condition in other respects). Otherwise you would need very accurate dimensions for the hole positions, and the drilling job would need to be done on a milling machine. This seems well beyond the normal scope of home conversions.
