You can remove and reuse the connector and wires off your Hobie battery if the new battery has the same terminals. See the following for instructions:
http://www.norcalkayakanglers.com/index.php?topic=89035.0If the new battery has different terminals you can splice the old connector as needed or buy a new one.
The Elite 9 draws just shy of 1 Amp. Figure 10 hours of fishing and that would be 10Ah. When sizing Lithium batteries I use a 50% margin for reserve and to keep from exhausting the battery so minimum 15Ah battery. Your original battery is likely 9Ah (or 7Ah if older). Increase capacity if you want to add lights, a bilge pump, or any other accessories in the future. There are several 15Ah and 20Ah Lithium batteries on Amazon with good ratings and reviews.
You may not find a battery with the desired Ah that is the same size as the Hobie battery (or close) at a price you want to pay. It will probably be a different size and shape. If so, you can just use long zip ties to mount it securely in your battery holder.
Peter