There is one for sale on eBay. Enter Sunfish drain plug in search. The seller is selling with horizontal hinge pin (good to have an extra) and deck drain plug. If you only want the cockpit plug try sending the seller a message. Til then, use a cork or a rubber stopper. You sound like you’re desperate to sail!
For what it's worth...I use a easy removalable rubber plug. Drains twice as fast as the cap and I just pull it as needed, when the boat is moving good. Tied the plug to a string, so it'll always stay with the boat. The old DePersia bailers were *fair to poor* at best in my opinion.