At any race that isn't using handouts.....you could swap in the correct arm without a problem....they SELL replacement arms for every trinity stocker, as well as the MVP/Core units....
What you CAN'T do is put a arm from another type of motor in your can. Example....if you have a monster, you can put a replacement monster arm in it....but you can't put a MVP or P2k2 arm in it.
At a handout motor race, typically the motors are marked on the can, endbell, and armature.....if the numbers don't match during tech....they will disqualify you.
Later EddieO