the weights defenitely right, now, tach that bad boy (all gassers are bad boys, but, thats a good thing )
I've got a ryobi 31cc that swings an 18x6, and its a hoss.
I'll tell ya how to avoid an electric starter altogether. This is how i start my homey 45.

Take the starter rope from the conversion, and wrap it backwards 3 times around the flywheel, then pull it off. Make sure that the rope is long enough to pull off 3 turns, and still be clear of the prop when its wound up (getting your knuckles skinned with a big prop really sucks... ask me how i know). My homey 45 hits on the first pull, then starts on the second.
Its cheap, and everything you need is right there.

Aerotrend makes a small pulley that attatches to either behind, or in front of the prop that will do the same thing for a cowled installation. Its about 20.00, if memory serves, but, if its open, the rope works just as well.