take a closer look at the honda challenge rules. You're limited to OEM parts below the deck... not just n/a. Basically, that means no forged aftermarket internals, sleeving, or anything like that.

Strictly following the rules, I think the best b-series you can come up with is an lsvtec using b16b pistons. Or rather, a b18c with ls crank, rods, and b16b pistons. I haven't checked lately to see if they've modified the rules since the release of the k-series mount kits, but if they have not, a k20a (jdm r) will definitely be the best you can do.