... unwisely at times used an incandescent light
I see from your profile page that you use a microscope extender to do your digitising.
Since you have presumably successfully digitised other slides using a different light source, is it not possible to re-digitise the problem images avoiding the use of the incandescent light?
If you have an extensive collection of slides it might be worth purchasing a proper slide/negative scanner or a scanner with slide/negative attachments.
You might even be able to outsource the job to a commercial scanning service.
... I don't have the artist's touch to use it.
Unless the area to be retouched is an empty monotone region, I doubt that any software can "paint" over it satisfactorily.