The quickest and easiest way to improve an old hand plane is to swap out the original blade and maybe even the chipbreaker. Not that there's anything truly wrong with the original pieces, other than a little surface rust and maybe a nick or two, but if you were to hold the new old and blades side-by-side you'll quickly notice a big difference.

The new blades are much thicker and therefore less likely to flex while being used and typically will hold their edge longer too. If there were any drawbacks to using the thicker blades it would be that they may fill more of the planes mouth than the original blade, but you may find in the case of a smoothing plane, this is a desirable drawback. And of course the cost of the new blades can set you back too, but it's a small price to pay for fantastic results.