Plywood is OK as a replacement so long as you can keep the water out. Any leaks will wick through the ply very quickly, since the grain runs flat across the sheet, and ruin the whole thing. Same with a plain board. It will work, but is not the best solution. This is one reason why end-grain balsa is actually better as a core. If water gets into a section, the grain will wick it in… all of 3/4″. Each block of balsa tends to get walled off from the others by the resin, which also helps to keep moisture from spreading. It’s also lighter.
Materials for your repair should not cost a lot. You should save about $500 doing it yourself.