Don't be too concerned about the timing. We run our engines at around 7000 rpm, so timing can be quite tame. However, the reduction of piston deck height is more damaging, so do not mill the case down, unless you are prepared to mill out the combustion chamber and restore squish space. This squish space can be checked with solder wire. It needs to be more than 0.5mm if you want your engine to survive.
Your ignition may limit rpm to 7000. Check that first. The flywheel has a patent number, can be checked at USPTO.org or .gov, or .com (forgot!). Anyway, the patent gives complete disclosure of how the ignition rpm cutoff (if present) works.