I've been wanting to apply the face that Strangetown's Bella Goth uses over her Pleasantview counterpart but couldn't find much about it online. Any ways to do this?

Sim surgery

Make sure to backup your hood first.
Open your hood in SimPE Tools menu> Go to Neighborhood and Sim Surgery
Then you should see sims on the left and and empty boxes on the right, one labeled "Patient Sim" and the other labeled "Archetype Sim"
Scroll through the Sims until you find the sim that you want to operate on and click to highlight them.
Once your Patient Sim(sim whose looks you want to change) is highlighted, click "Use" under the "Patient Sim" box. The picture of your sim should now show up in the box.
Scroll through the Sims again until you find the sim with the face you want to use. Highlight and click use they should now show up in the Archetype Sim box.
you should now see three options. "Face Only", "Makeup Only", "Eyes Only", "Skintone Only" Check whichever part of your sim that you want to change.
Click Surgery at the bottom of the screen. A warning may come up, but ignore it and choose "Continue" if it does.
Find the box that says commit and then save.

Open your game and teleport the sim over, the sim should be changed.

There doesn't seem to be an option across different Neighborhood, though. Should I export Bella as a BodyShop preset and make a dummy to use as the one I'll use for the face?

You could, sorry I am to use to playing an Uber hood where all the sims are available.

You can use the method similar to what people do to make plastic surgery result genetic - take the facial structure of one sim and import it over the other.

1. Tools -> Neighborhood -> Neighborhood browser -> [your hood] -> Open -> wait til it loads
2. In resource tree column choose "Sim Description" and in resource list find your sim
3. Down in Plugin view click More -> Open Character File
4. In resource tree choose facial structure
5. In resource list right-click on the Facial Structure -> Extract
6. Save it somewhere

7. Tools -> Neighborhood -> Neighborhood browser -> [the second hood] -> Open -> wait again
8. The same way find your other sim and their facial structure
9. Right click on it and choose Replace
10. Find the file you exported before, Open, Save everything

Should work, just make a backup beforehand just in case. Another option is to look up Alt Sim Surgery which works a lot like regular Sim Surgery, but allows to export and import Body Shop files instead of requiring two sims to be in the same hood.