Depends what the other group members want, there are a range of options.
#1. Have the tournament set before Mortimer's death.
#2. Have Mortimer's SP/MP transferred to a new character, either temporarily for the tournament.
#3. Replace him? (Don't recommend it, with the power houses Mortimer, Will and Jess (and Umi who's not participating), it makes it more balanced).
#4. Bull excuse like his spirit, ghost.
#5. Anything else Al or others can think.