I've got my SS mtb set up right now with a Truvativ BB and crankset (that's what it came with) running 42/18. I also have a spare crankset (also Truvativ) with a 32t chainring for bad winter sludge that a friend gave me.
But I worry that even the 42/18t is sort of low for riding on the city streets and want to look into a 50t chainring.

Problem is, I can't find any chainrings that work with my 4-hole crankarms in anything above 36t.
But I can't replace the cranks without the replacing the BB (can't find any Truvativ or ISIS cranks that would work with the 50t chainrings I have found)

Should I just replace my BB and my crankset with something like Shimano?
If I do that, will I be able to find compatible chainrings in small sizes as well as the larger? Or will I simply be swapping not being able to find large chainrings for not being able to find small ones?

Does anyone have a good setup they are running that they could point out?

Any suggestions would be muuuuch appreciated! Thanks!
ps. I don't want to fudge with the rear cog too much - ie, go to small (like 13t).