Are the plugs actually getting wet? Been thru these problems more times than i care to remember and its always something stupid and small and been double and tripple checked. Commer engine is quite easy to start even with a 'lazy' starter, Will it even attempt to fire? Any little puffs out the exhaust? If not my guess would be an issue with the timing. Perhaps the timing chain sprocket has come loose and is slipping on its shaft? might have enough grip to turn over slowley but under a bit more load it may be just spinning and sending the valve timing out of wack? I know its a long shot but i did have exactly that happen on an old car i had way back when.
Good spark and good fuel only really leaves timing issue of some sort.
Like BMM i am hanging out for the answer now!
Well...alright then, maybe just a little...