From what I've read on here, everyone says to get the biggest battery that fits in the box. Sounds like yours starts up just fine with the upgrades you've already done, I don't think you'll gain anything from a larger battery. Check the battery voltage a full throttle, should be 13.5ish. A bad regulator ground can cause it to malfunction, so check the grounds before calling it dead. Your probably already aware, but there is a link to the engine manual in the tech section. I'm sure it has regulator testing procedures in it, my phone won't let me verify.
http://www.onlycubcadets.net/forum/s...ead.php?t=4660
Bill