Can I speedrun the game using USBloader ? I didn't have any other solutions for playing the game.

You're allowed to speedrun using USBloader but you're not allowed to submit to the speedrun.com leaderboards per the rules.

"All emulators, MIOS modifications, or homebrew applications capable of running Gamecube ISOs (Dolphin, Dios Mios, Devolution, Nintendont, etc.) are banned. The use of a backup disc is banned. Launching the game (via disc or otherwise) from Nintendont is banned (as it seems to make the game run faster than normal)."