Triathlon on single-speed

It's a very flat 20K, loops around Dorney lake with lots of straights.

Do I have to man-up and replace the chainwheel (and crankset?) with something teethier 46/48?

You need to man-up all the way to 52t/53t chainring if you're going to be vaguely competitive around Dorney. There are 10 mile TTs at Maidenhead (not as flat as Dorney, but flat enough) every week which would allow you to try different gearing options. I've got a bunch of 130pcd rings from 51 to 56 teeth if you're over this way and want to experiment.