Hi,
well, basically just fill in what is missing to the best of your ability. using triangle stock, filler and generous amount of epoxy to put it together and you should do fine.

try to get a thicker piece of plywood for the firewall if you are concerned with the 150 on it, and beef up from there.

hard to give advice without seeing it in detail. best of luck. CG ultimates are meant to fly, not crash. for the most part.