I've got a chewed out thread where my grubb screw holds the fcs1 fin in place.
What's the cheapest and easiest way to repair this.
What resins glue? Any extra hard stuff available at Bunnings.
Then what size drill and tap, I would think they all would be a standard size.
Thanks in advance for any advice.

Bigger grub screw.Used a legrope screw once as an emergency repair worked well .

Depending on what the exact problem is, I've had success from simply buying much longer ones. You can get them just about any length you want from a good stainless steel fasteners supplier.

I've also used Harrow's solution and it worked.

I would also go bigger - next size up in metric would be just big enough to cut a new thread (eg 0.5mm bigger) as FCS is imperial