Well, on consideration that a 42t chainring will give up to 92 GI (42x12), I see no need for a different chainring.
I'm using a 42x14 for about 79 GI, which puts me at a cadence of about 60 for 15 mph. Accordingly, I can ride 30 mph, but I'm spinning at the top of my range.

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It's not exactly reccomended, but you can rotafix (google/search) the cog if you have a lockring tool.