Windmill Class Association

I am the proud new owner of a 30-40 year old Windmill. The boat is in great shape. Painted white exterior varnished wood interior with a wood mast and boom.

I noticed the interior of the hull has a bubble in the wood. I looked underneath to find a 3-4 inch hairline crack maybe a little bit bigger than a hairline. Do I need to completely replace the wood? Any suggestions? Can it be sanded fiberglassed and repainted on the belly? If so how do I go about doing that?

Thanks for your help!!!