If your freewheel is in good working order, also consider swapping one or more sprockets to properly complement your chosen chainring combination.
Many people regard a freewheel as a monolithic unit, but in truth it is an assembly, and customizing the combination of sprockets was just as common BITD as it is on cassettes today.
What you need to embark on this adventure is a couple of sturdy chainwhips (perhaps these can be borrowed from your local bike coop or friendly LBS) or 1 chainwhip and a freewheel vise of some sort (can be improvised). Be prepared for possible destructive removal of the small sprocket. Sometimes these are just on too tight for non-destructive removal, but that's only happened to me in one out of dozens of freewheel teardowns.
Replacement sprockets are readily available. Preferred acquisition method is via teardown of a donor freewheel. Individual sprockets are also available on Ebay, or can be traded with members on BF. For interchangeability, it's best to stick with one brand/model, until you learn the nuances.
As you teardown you will quickly see that there are specific positions for sprockets as they go from small to large, so the incoming sprockets have to be selected for the position for which they are wanted, as well as their tooth counts
If all this seems too much for a beginner, you could try Pastor Bob's freewheel service. I'm sure he could fix you up with a customized freewheel at reasonable cost.