most aftermarket exhaust have a S/A insert option. But most are loud, like the Jardine. Check with MRD to see if they make a 250x pipe. Full system is like $430
I dont know your skill set, but a new cam would be easy to install. Just have to check valve clearances and possibly shim.
Airfilters wont do much if anything.
Porting and hicomp pistons require removing the head and cylinder, so you also have to buy a new gasket set there. good way to make some power there, but will cost a bit more $. Dont know if the 250 is like the 450 where you can run the R head for improved power. Sure you can get a complete head with valves in the $500 range new.