West Marine's Group 27 1050MCA Starting Battery is the Deka 27m6. It's not a clone, just a re-branded Deka. You can find them on sale occasionally.
It's hard to find other Group 27's that will meet the Yamaha specs. The 27m6 was designed specifically for that purpose. They are actually dual purpose batteries (hence the high reserve capacity) and are designed to be practically maintenance-free. You rarely have to add water to them. Their lead plate construction is also particularly robust.
Yamaha specs the high reserve capacity since the big engines draw more current at low rpm than can be produced by the alternator. They draw off the battery under those conditions. That's why they won't continue to run with a disconnected or dead battery. This is really only an issue if you do a lot of trolling with electronics running. Think of people who cruise around at idle speeds with powerful speakers blasting. For most purposes, any marine starting battery with sufficient MCA will be OK. Once the engine is running at 1500 rpm or so, the alternator will keep the battery charged.
