Square taper cranksets are common, you can find used ones or new ones fairly cheaply with either mountain (~42t) or road (~48-52t) chainring sizes.
Some Considerations for Switching from Mountain to Road Crankset:
number of gears
Mountain cranksets generally have three gears and road cranksets usually have two. To switch to a road crank with two gears, you can just adjust your front derailleur, including the limit screws, position & cable tension, and only use two gears of your three-speed shifter.
derailleur position
To accommodate the bigger chainring size, you will need to raise the derailleur on the seat tube so that it is still 1-3 mm away from the outer teeth while shifting. You will probably also need to tighten the low limit screw quite a bit, as there is no more very low gear. Finally, you will need to reset the cable tension. Depending on how much you moved the shifter and the condition of the cable, you may need a new front derailleur cable to make this work, but that's a cheap upgrade that is usually worth it just for the sake of reducing friction when you shift.
chainring clearance
One thing you will need to be careful of is that there is enough clearance between the chainstay (the tube parallel to the chain) and the chainring. If it's pretty close to the 42t chainring now, it might not fit the larger chainring (without, that is, extending it with a longer bottom bracket, but that raises additional complications with the chainline, i.e., you want your chain to run as straight as possible). But, it might fit just fine, in which case that would be a pretty easy swap to make.
other considerations
If choosing a used crankset, make sure to find one that does not have any visual wear around the teeth (should not have an asymmetrical tooth shape) or the inside of the square taper. There should be no apparent wear in either of those places or it will lead to problems fairly quickly. Otherwise there are lots of good used options and new options in the sub- $50 range.
