Roscoe just put out a video a few days ago on changing a spar on an AA-1. The process is the same on an AA-5. https://www.youtube.com/watch?v=9bFbHNijHg0

Now finding a replacement spar is the issue. No new spars are available. I had heard that True Flight was working on getting new ones made, but as far as I know that has not yet happened. Best bet would be to check with Fletchair for a used serviceable one. If you purchase a used one, make sure it has the time documented on it as it is a life limited part (11,500 hours I think?).

Good luck on this.

Here is the section from the FAA AA-5 Type Certificate on Life Limited parts.
I don’t know if Australia has any different requirements.

Service Life Limit Information with respect to service life limited parts on this Model is contained in the
applicable manufacturer’s service manual, “Model AA-5, AA-5A, AA-5B, AG-5B
Service Manual, ” Section 3-00, “Service Life Limited Components.” Service life limits
appearing in this manual may not be changed without FAA Engineering approval.
Service life limited parts must be retired in accordance with the following schedule:
Component Part Number Service Life (Hours)
Inboard Spar Assembly 5102310-503 12,000
Wing Spar Assembly 5201004-501 12,500
Wing Outboard Spar Assembly 5201189-501 12,500
Shoulder Bolt 901044-2 7,250