What you're looking for is a Yuasa YTX20CH-BS or equivalent - almost all battery suppliers cross-reference to the Yuasa models. This spec is important to use IN FULL, since the terminal location is the opposite of most MC batteries.
This is the same battery spec as later Norges - in fact, it's the same battery spec as on lots and lots (and lots!) of Moto Guzzis.
The Yuasa batteries are very good, the Odyssey AGMs are somewhat better (though I'm not sure enough so to justify the extra cost). Some third-party AGMs are in the same class as the Yuasa or even a bit better, but I don't have personal experience with them.