"Erratic Flight Behaviour" doesn't sound to me like too big a fin or rudder.
Sounds more like the CG too far aft or radio problems, or something is loose or flexing.
A bit of structure or a control surface or pushrod cracked and moving about would be my guess.
Shame it is too late to find out and sorry to hear about the damage. Trashing an Irvine 53 is a tragedy.