Basically there are 2 circuits in the petcock, 2 holes from the tank. the selector switches which one is open to the outlet that goes to your fuel line - on, reserve or both closed = off. The difference is the on draws through the little tube a few inches off the bottom, while reserve draws straight from the bottom. The risk running on reserve is duh, you don't have any reserve if you run low on gas, but bigger risk, since it draws from the bottom, is whatever clogged the ON channel is likely to clog the reserve. You can pull the tube out, then on will behave same as reserve. You probably don't need to replace the petcock, usually it can be cleaned out. but sometimes it's clogged up behind the selector lever, and removing that can sometimes detroy the rubber gasket in there and then it'll always leak, usually easier to replace the petcock than find a replacement for that gasket.
First thing I'd try is to just blow back into the tank really hard. you might hear it bbbling inside, that means you've cleared the clog and it'll work at least short term. This is more of a roadside fix though than an actual permanent solution.
To really get it, drain the tank and pull the petcock out, flush some gas through the tank with he petcock off to get whatever crud is in the tank mostly out. Then blow through the petcock from the fuel line and try'n clear both passageways, set on ON and on Reserve. Then if nada, can try'n blast through the petcock with carb cleaner - same idea.
If all that fails, you could try removing the selector lever to access the fuel channels inside. If the gasket breaks you can clear it all out and just use a rubber o-ring in there. It will make it so both channels are always on and the lever does nothing, can't turn it off, but it will not leak and should flow gas fine. Or replace the petcock at that point.