Don't do the Kenji missions!
Seriously, you don't have to. Just do everything else and work your way to Donald Love and there is a mission there where you kill Kenji.
"I drink your milkshake! I drink it up!"
You don't need to do a lot of things to finish the main storyline. This includes telephone missions (Marty Chonks, El Burro, King Courtney, and D-Ice), taxi and emergency vehicle missions, exporting vehicles (both garages and the crane), rampages, unique jumps, RC and offroad missions, and collecting hidden packages.
But avoiding any of them would make 100% game completion unreachable without using a trainer modification to falsely indicate that they'd been done. Given that these side tasks make up nearly half of the game's percentage (and are generally more challenging than required missions themselves), skipping Marty Chonks and Kenji yields nothing but a messed-up save file.
100% completion is possible to aquire after you've played through the main storyline...provided that neither one of the skipped tasks involved Marty Chonks or Kenji...and provided that you didn't use the "new game" option on a PS2's memory card that already has had a beaten "RUMBLE" mission saved at any time in that entire memory card file.
Spoonfeeder Glitch avoidance list:
Marty Chonk's last mission "HER LOVER" must be completed before you finish "LAST REQUESTS" for Salvatore.
Kenji's last mission "SMACK DOWN" must be completed before you finish "WAKA-GASHIRA WIPEOUT" for Donald Love.
The "New Game" option must never be used on a PS2 memory card file on which D-Ice's last mission "RUMBLE" has ever been completed on any of it's save slots (that's right, the entire memory card file is affected). To avoid getting this glitch at all: erase the whole memory card file from the PS2 browser menu (if a save slot has ever had D-Ice's missions completed), begin a new game, complete the intro mission, save it to a slot, and never overwrite it. Use that slot whenever you want to start a "new" game.