I'm no Tarawa scholar, and I'm unfamiliar with Rapid Fire, but your question piqued my interest, so I did some Wikipedia reading and net surfing.
1. You're probably going to want to step the rules down from Brigade to Battalion level, based on the principle deployment units. Personally I think a game of this type would work best at Company level for the infantry units and platoon for the specialty support units.
2. Given the static crust nature of the defenses, and the complete failure of the Japanese field phone system, this is a game that would probably lend itself to having all the players run USMC, with a randomized or GM moderated Japanese side. The Japanese stuck to their bunkers until forced to retire into the trenches behind, and fight on until things deteriorated to banzai time. Decision Games have a D Day at Tarawa solitaire game that might be a real boon to your efforts. If for no other reason than getting the game map could well be a treasure trove.
3. The Day 1 Northern Red Beach assault waves encountered horrific problems. The expected high tide proved to be a "neap" tide, far shallower than expected. The LCVPs could not clear the outer reef, so the marines had to debark there and wade ashore. The LVTs could crawl over the reef, but were subjected to murderous fire on the approach to the "beach."
Much of the Red Beach shore was not beach, but a tall coconut log sea wall. While this did offer a precious defilade, it also obliged the marines to essentially go "over the top" to move inland. The defilade was imperfect until the marines cleared the snipers from the Burns Philips wharf. Was there anyplace in the Pacific that didnt have some kind of Burns Philips presence? You should plan to build in wading movement under fire, and movement over the sea wall fully exposed.
You should probably plan to completely dissolve unit cohesion into an almost man for man count of personnel reaching the shore, then dice for officers trying to reform "pick up" commands of infantry, and dice for possible assembly of support elements. Think Saving Private Ryan.
As it happened, the marine's advance looks to have occurred primarily from the center of the Red Beach shore, where there was no seawall and the LVTs could actually debark on dry land.
The second day landings on the western Green Beach progressed more conventionally and saw armor finally getting feet dry.
4. Of the 5000 man Japanese garrison, about 2900 were SNLF. The balance were labor troops. Of them, some 1200 were Koreans. I leave it you sort out the relative combat capabilities of each of the 3 groups. It bears noting that only 129 of the Koreans survived. The Decision Games offering could probably help here.
5. If you wanted to deconstruct and start from scratch, I'd suggest using a sectored game map much like the Board Game Storm over Arnhem. Each landing craft approach lane would start at the reef and have several "steps" to the shore. The shoreline would be subdivided based on Japanese unit boundaries with bunkers at the front and a trench system behind. The marines could only enter a beach sector once some bunkers were overcome. Further bunker fighting would have the marines enjoying flanking advantages. Marines move, Japanese fire, Japanese move, Marines fire, Naval gunfire support, if available, close assault.
6. Of course a rivet counting purist would argue it should be called Betio, not Tarawa. Betio is to Tarawa as Gettysburg is to Pennsylvania, or some such blather.
Don't ask me how to pronounce Betio. Online you can get at least 5 different ideas. Bay chee oh; besho; bittio; whatever
Just some ideas.
And people say nothing good comes of insomnia.
