Small Engine Repair (updated in OP)

15 Horse Briggs on a MTD Riding mower that my dad has. It had a weak battery for a while but he would just jump it off when he needed to use it. He tried to jump it off one day and it wouldn't turn over, so he figured the battery was completely fried. He went out and bought another battery and when he tried to start it, same thing. It tries to turn over but it acts as if the new battery is dead too(he is on his 3rd battery now so battery isn't likely the issue). It will barely turn over when you jump it off. I know the motor isnt seized up because you can grab the shaft and turn it by hand. Also, I noticed when trying to turn it over that the ground terminal on the battery got really hot, and I only turned the key for a second (not jumping it off that time).

I am trying to get it running for him before the spring gets here so I am looking for some expertise. It seems as if something is grounding out, causing a loss of power to the starter, or maybe the starter itself is getting weak. The motor is roughly 8 years old. Any insight on this issue would be greatly appreciated.

Update 3-18-14: I brought it home and started digging into it. I tried jumping it off with my mower but the starter couldn't get the engine to turn. I pulled the sparkplug and a bunch of dirty gas came out of the spark plug hole. I turned it over with the spark plug out and it spun freely, but as soon as I put the plug back in it can't spin. It appears that the starter is too weak to spin with the compression?

Update 3-27-14: This thing is driving me nuts. Valves were out of adjustment which didn't let the compression-release engage. Got them adjusted and it fired right up (without needing to be jumped off). I ran it for a minute then had to shut down for the evening. Today I was going to change the oil and mow my yard with it but it wouldn't start again. I pulled the valve cover off and checked them, they seemed fine. I jumped it off and it ran great for a few minutes, so I shut it off to put the hood back on so I could mow the lawn. I went to fire it back up...nothing. Wouldn't even jump off this time. I tried putting my battery from my mower in it and it wouldn't turn over (my battery/mower are practically brand new). I put my battery back in my mower and it was dead!?!? I literally tried starting the other mower 1 time with my battery and it killed it.

What is wrong with this thing? I am guessing the stator is shot since it killed the battery after running for a while, but what would make it kill my battery instantly?

TLDR; this thing is cursed.