Having received the IPC and reassured myself there was nothing unusual in the way the flywheel is retained, I set about trying to remove it today. Even after a couple of rounds of heating and penetrating oil, it was soon clear that simply applying a spanner on the two 'flats' of the flywheel nut would do more harm than good, so I gripped the nut firmly in the bench vice and heaved on the flywheel for all I was worth - nothing. Next I made strap wrench with a bit of webbing and about a foot length of timber - nothing. Replacing the timber with rather over a yard of angle iron, the springy feeling gradually became more 'plasticy' - either it was moving or I had succeeded in wringing the end off the crankshaft! Fortunately it turned out to be the former and, had someone not attempted to drill out the screws which hold the starting rope sheave on, the self-extracting feature might actually have worked - as it was the flywheel taper succumbed to another round of heating and penetrating oil.
Evinrude were proud of their magneto coil - a 1929 catalogue states that it will withstand complete immersion for up to six hours - well they must have got something right as applying 1 1/2 volts to the scabby looking coil which was revealed when the flywheel finally relinquished its grip produced a pretty respectable looking spark :-)
Still much to do, a frustrating sort of engine in that all the parts that are not actually broken seem in pretty good order but it has clearly received the attentions of a real gorilla at some time - how does one set about re-attaching part of the base flange to a cylinder?