I purchased an English reproduction copy of this game for the Dreamcast from Nights Into Dream, and I was wondering if I could use it to speedrun on this board, or if I'm only allowed to use the official Japanese game.

you are allowed to run on the dreamcast.

you can use it but i think its probably slower than an official disc

It probably is, but I don't have a means of playing the Japanese version on console atm (the game was never released in English on the Dreamcast in case either of you didn't know).

It was released in Europe, I have the english version on my Gdemu

Interesting, I did not know that