You need to know the BCD of the chainrings in order to find one that fits. Yes you can probably find a 42 that will fit, but then the drop from the 42 to the 24 will be inordinately large and will shift like crap. You could just put the 42 back on the outer. With that said, a 38/11 gives you 25mph at 90 rpm in the 11t cog in the back . . . do you really need larger gearing?