I don't think "shokun" is polite enough myself.
多数の人をさす語。主として男性が対等以下のものに軽い敬意をもっていう語。
Regarding the "saying"... hmm... It's very hard to just say that outright like that and expect the Japanese listeners to understand your feelings exactly. It might be better to say something like "today we meet in the spirit of the saying 'xxx' " or something. Just dropping a rough-style classical-Japanese
samurai phrase into your remarks may be confusing. Need a native speaker though...
JKA is 日本空手協会, branch is 支部 so altogether
Amerika Karate Kyoukai Kariforunia Shibu
is the name of your club.
I would say (building on above posts)
Karateka no katagata, kankyaku no minasama, Amerika Karate Kyoukai Kariforunia Shibu no senshuken taikai e youkoso
空手家の方々、観客の皆様、アメリカ空手協会カリフォルニア支部の選手権大会へようこそ
The "saying" I don't really have a clue sorry.