Chain line adjustment when replacing triple crankset with single
Assuming the chainline is spot on as it is now, you could remove the inner and middle chainrings, and place your 48t outer ring in the middle ring position. This will most likely require new chainring bolts intended for single ring use (they are shorter).
This will only work if the middle and outer ring share the same bolt circle diameter - BCD (most triple cranks work like that, but I've come across a few where the inner and middle rings both had a smaller BCD than the outer ring.)

If above solution doesnt provide the chainline you want, you could replace the bottom bracket. Shimano makes a cheap and (in my experience) reliable bottom bracket for square taper interface: The BB-UN26. It comes in various spindle lengths and for both 68mm and 73mm shells.