Here are some photos of what one of our customers did with a small ID Lyc flywheel to mount magnets on the rear face. These might give you some ideas.
Tapped 8-32 holes for magnets
Custom Hall sensor mount
The Hall sensor can be mounted anywhere around the engine that you have room and that position determines where the magnets are placed in the flywheel. Aim for about .060-.090 air gap.
Drill magnet holes with #29 drill and tap for 8-32 Allen set screws. We can provide you with the set screws and new magnets if required.
You can find general info on how the magnets should be arranged here: http://www.sdsefi.com/faero2.pdf