If it's an O'Day 1986 my guess would be that it is an O'Day 222.
D & R Marine shows the prices for centerboard and parts to be around $600, or so, not including labor.
DR Marine Product List
It could be a fun project, but you'd have to see what broke in the centerboard trunk that allowed it to fall off in the first place. If the board broke that's do-able; if something in the trunk cracked that could compound the problem.
That is a terrific boat, fractional jib, keel with centerboard line uphaul (as opposed to the clunky crank on the Catalina), and you can easily put the traveler just aft of the cabin on the step.
Great entry level boat, day sailor and weekender.
Still, as SecondwindNC said, THAT BOAT could be a losing proposition. You should have someone from this site look at it with you or get a survey, sense of the market kind of thing.
Where is it located?