Here are my suggestions for what phonemes you can substitute for each japanese kana.
あ a
い yi
う wu
え "ei" CV and "ie" VC (sometimes written as e0)
お "ou/ong" CV and "o" VC
ん use the various "an in ang ing" etc. endings
か ka
き short "kei" with long "yi"
く ku
け "kei" CV and "ie" VC
こ "kou/kong" CV and "o" VC
きゃ short "kei" with long "ya"
きゅ short "kei" with long "yu"
きょ short "kei" with long "you" and "o" VC
がぎぐげごぎゃぎゅぎょ similar to K
さ sa
し xi
す su
せ "sei" CV and "ie" VC
そ "sou/song" CV and "o" VC
しゃ sha
しゅ xu or shu
しょ "shou" CV and "o" VC
ざ za
じ ji
ず zu
ぜ "zei" CV and "ie" VC
ぞ "zou/zong" CV and "o" VC
じゃ zha
じゅ ju or zhu
じょ "zhou/zhong" CV and "o" VC
た ta
ち qi
つ cu
て "tei" CV and "ie" VC
と "tou/tong" CV and "o" VC
ちゃ cha
ちゅ qu or chu
ちょ "chou/chong" CV and "o" VC
だ da
で "dei" CV and "ie" VC
ど "dou/dong" CV and "o" VC
な na
に ni
ぬ nv or nu
ね "nei" CV and "ie" VC
の "nou/nong" CV and "o" VC
にゃ short "ni" and long "ya"
にゅ short "ni" and long "yu"
にょ short "ni", long "you", and "o" VC
は ha
ひ short "hei" and long "yi"
ふ fu
へ "hei" CV with "ie" VC
ほ "hou/hong" CV and "o" VC
ひゃひゅひょ similar to ny, using "hei" instead of "ni"
ば ba
び bi
ぶ bu
べ "bei" CV with "ie" VC
ぼ bo (this sounds like "buo", unavoidable)
びゃびゅびょ similar to ny, using "bi" instead of "ni"
ぱ pa
ぴ pi
ぷ pu
ぺ "pei" CV with "ie" VC
ぽ "pou" CV with "o" VC
ぴゃぴゅぴょ similar to ny, using "pi" instead of "ni"
まみむめもみゃみゅみょ similar to p
や ya
ゆ yu
よ "you" CV with "o" VC
ら la
り li
る lv or lu
れ "lei" CV with "ie" VC
ろ "lou/long" CV with "o" VC
りゃりゅりょ similar to ny, using "li" instead of "ni"
わ wa
I've attached a USTx that you can use as reference.