If you're still trying to get this going, you should take a look at the Anime d20 System Reference Document, which is based on combining BESM with the rules for D&D v.3.5. The best part is that is free. The company that made BESM d20 doesn't print the book anymore (I think), but you can get the BESM d20 Revised Edition through White Wolf's store on RPGNow.com (or DriveThruRPG, if you prefer [some do]), if you want something more polished. There are other BESM d20 books available through that site including a Monstrous Manual, Stingy Gamer Edition, Mecha book, Military Vehicles book, and character folio, as well as Silver Age Sentinels books.
There were a lot of other BESM d20 books, but I don't know how many of them are still available. In reality, you should be able to create a BESM d20 Ragnarok game with just the Anime d20 SRD. It's a lot cheaer than investing in tons of 5E books, which are VERY expensive. Don't get me wrong, I love the ruleset, but I don't buy the adventures. It's too much money for one book, IMO.
Of course, you've probably invested in most of them, so use what you like. Don't forget to check the D&D Official Homepage for more 5E content, especially Dragon+ and the Unearthed Arcana articles. There are lot of good options there, and if I were you, get in contact the D&D staff and let them know about Dungeons and Dragonlarks (assuming you haven't already).