Can I brush epoxy over just the tacky spots instead of repouring?

Yes, as long as the spots are hard with only a slight tackiness on the surface. Mix up a very small batch of UltraClear Bar & Table Top Epoxy and paint over those areas with an artist paint brush. Dip the brush into the center of the container, not the sides, so you're applying fully blended epoxy; material from the sides of the container may be unmixed. The same approach works for a small tacky area where you used the heat gun.

The trade-off is appearance. A spot fix will likely leave a surface that isn't seamless, with visible lines between the new and old epoxy. The only way to guarantee a seamless finish is to re-pour a flood coat over the entire surface at once. Which route to take is up to you and depends on the project.

If the spots are soft and wet rather than just tacky, brushing over them won't work. Scrape or cut out as much of the soft material as possible first, using denatured alcohol or acetone to help, then recoat.

Either way, the new epoxy must be measured at exactly 1:1 and mixed 100% completely. As long as it is, it will cure hard and clear over the old tacky areas.