If writing a fic involving magic, it is best to have a clear and established magic system set up. Still working on it but I do have this:
1. Shinto magic: Ranma will know how to create some o-fuda (Shinto wards), know Purification rites (just about every Japanese person knows some), and commune with spirits/kami. Ranma is familiar with Shintoism. He hunts monsters occasionally, deals with spirits, and has gone to shrines. Wards have been used repeatedly in the manga (on panda ghost, oni, to trap Happosai in cave). Ranma has met and touched Kami (the ugly divine horse for instance). Ranma has to have picked up some of this. He has communed directly with Kami (fickle animal Kami are fairly common) , dealt with dangerous demons (oni which are kami), spoke with/dealt with spirits, etc. For fic purposes Ranma can ask the Kami for help/power but he isn't going to do it often since he would prefer to use his own power (it's also far more reliable). Kami do what they want to, may want something in return, and may want to bother you for entertainment purposes. While many Kami are good they rarely are perfect and there are Kami that would do more harm than good Kami of evil, tricks, sickness, and destruction for example.
2. Buddhist magic: Unsure if I will have this do anything since the magic in anime/movies is usually obtained from saying the Surtas. Ranma may know them but don't have anything planned for them. He likely is both Shinto and Buddhist since many Japanese people are both Buddhist and Shinto. Ranma has seen the river between the land of the living and the land of the dead which is Buddhist.
-Side note - Religion is not a big part of most Japanese peoples lives. Average Japanese person only does religious rituals at ceremonies at Births, Wedding, and Funerals and occasionally going to festivals or Shrines/Temples on days like New Years. There is a saying that goes Japanese people are Shinto at birth, Christian at marriage (they like western weddings) and Buddhist at death (Shinto doesn't really cover death so most people turn to Buddhism for death).
3. Voodoo: There will be a character who does voodoo but it will mostly be made up. Ranma will know at least a little about Ushi no koku mairi which is Japanese curses similar to Voodoo (often called Japanese voodoo) since Gosunkugi attempts to use it on Ranma repeatedly (though really he should be doing it between 1 and 3 am during the time of the ox if he wants it to work and at a shrine with the doll looking towards the north-east), but will have next to no knowledge on the type used in this fic. He will attempt to combat it with Shinto. There are parallels the loa and the kami are similar type beings and gris gris bags and such are similar to shinto charms like ofuda though there are many differences as well. I'm thinking of having the fics antagonist using Voodoo simply because it is the western religion most similar to Shinto and it is already associated with possession (the Loa possessing and giving power) and zombies.
4. Magical items. In the Manga though not so much in fanfiction Ranma frequently uses magical items. In the fic Ranma will have some he has obtained from Cologne, stolen from Happosai, and purchased/obtained through other means (he came to Nerima with magical items in his possession girl repellent, dragon whisker, and may have had more that just didn't have a practical reason to use). The antagonist will have magic items as well. Every magical item will have limitations.
5. Jusenkyo - Ranma will have his curse and may make use of some instant spring waters but unsure how much use it will have in fic.