The original rudder on the 1982 and older Javelins (as well as the Day Sailer, exact same rudder) will not float, the newer ones (about 1983-89) might float, since O'DAY changed to a new design that had an aluminum top and wood blade for those years. The aluminum might be too heavy for the wood to support?
As Joe has said, Rudy at D&R Marine has the original rudder and most likely the newer one, not cheap! Also, you will need to change the mounting hardware if you switch to the newer version (original wil lfit with out mods).
Along with the suggestion that Joe had for a retaining pin (rudders from D&R have that hole already fitted and include pin.), I'd suggest a short lanyard that attaches to the rudder and is just long enough to allow the rudder to be taken on/off while the other end of the lanyard is attached to the boat. I had one of those on my old Widgeon and it was very handy!
Finally, the best advice to avoid repeating that capsize would be to ALWAYS sail with the mainsheet in your hand, use the camcleat to hold the tension, but hold the sheet in hand ALWAYS. That way, if a gust hits, you can release the mainsheet easily to spill the wind and prevent a capsize. I'ver used that technique successfully on my DS II for 17 years and on my Widgeon for 7 years before that.