Always a good idea to try positioning new bare flywheels with a length of wooden broom handle in place of mainshaft to check all is approximately as calculated. Not least the available distance between them for the conrod !
A crack in the originals can be clearly seen close up running towards the mainshaft from the oil feed outlet on the inner wall . Probably would not get much worse if only subjected to light road use but sprinting is too much of a risk.
Norton simply used cast iron for their road going OHV singles so it is to be expected that after a half century of use this might be weak spot to reveal itself.
Found a use for old CD or DVD discs. Download a timing disc from the internet and glue it on. Although in theory the bigger the diameter the more accurate the reading I find that it is more easy to mark zero for TDC without making up a bit of stiff wire and finding a place to attach it.