Cutting fiberglass is easy, you can use a saw, a dremel, a grinder, doesn't matter as long as it works. Making a smooth edge is not needed, in fact a rough edge will soak up the resin better and provide a better bond. For small areas, you can substitue epoxy for resin, it will bond just as well and it will be ethanol proof. For a fuel tank repair, make sure the patch is at least as thick as the base material and if you have access to the insides, then you can put a bigger patch to secure the patch to the tank. Everything must be ground down to make sure you are not rying to get resin to stick to crud.
After you are done, sand with finer and finer grits, then paint and clear just like the old stuff. If a part has never been redone, it could still have the gel coat on the outside which is a tough plastic like material. You will never be able to get the same finish if you try to fix a small area, so a paint job may be in order for a top quality finish but I am not a painter so don't take my advice for a paint job.
There are many video tutoprials on fiberglass repairs on YouTube, well worth taking a look before starting.
Take pictures and write it up.
Jean