Is the Shimano FC 175 TY301 crankset still available. If not, how do I choose a good replacement without changing anything else?
That model crank is listed as a current component in the Tourney range. See this page on the Shimano site: https://productinfo.shimano.com/#/lc/tourney/3x7
The key to square taper 3-piece cranks is the axle length of the bottom bracket. Different model cranks require a different axle length to get the correct chainline (distance of chainrings to center line of frame).
The Tourney TY501 crank is direct replacement, the TY801 requires a different bottom bracket axle length, so the bracket has to be replaced with the crank. You could also upgrade to a higher level Altus M311 crank but again that requires a different axle length
Shimano's specs are here if you want to check them out: https://productinfo.shimano.com/#/spec/MTB/Front%20Chainwheel. Search for 'FC-TY301' and look for the BB axle length row in the table.