ARC now has a billet flywheel for the flathead that will allow you to use the new PVL ignition from B&S. We have seen substantial gains with this coil and flywheel over the standard (old) coil and ignition.
The new flywheel (#6617) weighs 3.85lbs. It has adjustable timing and is SFI certified.
Although this new flywheel has a larger diameter than a 3hp flywheel, the rim weight of the new flywheel is considerably lighter than the 3hp models. This means that acceleration with the new flywheel would be about the same as a 3hp model. BUT, the PVL ignition used by the new flywheel will give you gains beyond what you could ever get with the 3hp models with standard coils.
The reason this flywheel must have a larger diameter than a 3hp flywheel is because the PVL coil is designed for a specific diameter and appears to be sensitive to any deviation from that diameter.