How much juice does it take to fire a bike?

So, I'm lazy, and I've been super busy with my new job. I finally had the time to get the bike out of storage. Of course, the battery is stone dead. It's one of those Harley AGM sealed batteries, and I got it used(free) a few years back. Aside from being lazy, I'm a cheap bastard, so I'm probably not gouing to buy another battery unless I absolutly have to. So here's my question. Will a marginal battey make enough electricity to kick start a '77 Shovelhead with points ignition? How much juice does that take anyway?