If you use something that has a "noise removal filter" with the option of JPEG artifact removal, that may improve the images near edge or colour transitions.
Save them as either .png or use .jpg but with quality at or near maximum.
The jpg image format uses "lossy compression" to reduce the file size - but causes loss of quality as well, when the image is compressed.
As long as the image size of the originals was somewhere near 3000 x 2000 pixels (or more), they will be fine when printed at normal photo.
Normal 35mm colour film typically has around that equivalent resolution, so photos printed from digital images of around that or better should be as good as film camera prints.
If the old images are really low resolution there is not much you can do; they just do not contain the same data a higher resolution photo would.
If pixelation is obvious you can possibly smooth it out a bit; resize the image to an exact multiple of its original size, eg. 8x both ways. Do any noise removal and adjustments to brightness & contrast at that scale.
When you are done, reduce it to a more practical resolution, again an exact fraction (1/2 or 1/4) of the expanded image size. That should give some pixel smoothing without messing up the original detail too much. eg. If it were 1024 pixels wide, change to 8192 then to 4096 or 2048 after processing.
Never overwrite the original images, so you can always go back to those if something goes wrong!