doubtful you will ever find pieces/parts for that tower. betting it hasn't been made in 10+ years. I'd suggest a call to skiers choice as they would have tower vendor info. my first guess would be roswell as that's who made all the racks and speaker/light pods back in the mid-00's for skiers.

if it was my boat, as holdmybeer suggest, remove that section of the tower and find a good machine shop to create a new plate and weld it to the round tower tubing. rather than fix what's there I can't imagine a new plate welded would run more than 2-400. that's going to be over half what a new tower would run even on sale without getting into the costs of gel repair.
good call on checking the other side for cracking as well.

tubes would be my first guess but boarding/foiling can put some stresses on a tower year after year and guessing that boat is 15+yrs old.

good luck...