I'm in the middle of that conversion on one bike myself. Not much of a conversion: I'm only keeping my crank arms.
I chose a different chain ring for another bike that came out of the box 1x11. It's not that hard if you're conversant in gear ratios. I'm medium-comfortable with gear inches but if you're only interested in one wheel size, dividing the chain ring tooth count by the cog tooth count tells you everything you need to know.
So run a gear chart for your old drivetrain. Now, in real life, which gears do you get the most use out of? For me it's been my granny, with the top few ratios pretty much being decoration. So I chose a chain ring that gave me the same lowest gear ratio. That's worked out pretty well, although I find I'm in my highest gear somewhat frequently now, with the narrower range I get from a 11-42 cassette and single ring. That hasn't really been a problem, although I can see how if I was more competitive in enduro, I might want a wider range so I could raise my highest ratio a bit.
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