What sort of regulator did you convert it over to?
If you're still running the stock charging system, a lithium battery is a bad idea. Modern battery tech does not mix with old school charging systems. Even with a modern regulator, some battery companies claim their lithium batteries should not be used on old school bikes because of the variation in the charging voltage, even if it is under the maximum allowed.
At the very least you need a MOSFET regulator. The re-start antigravity batteries apparently have a BMS built in but even that isn't a fail safe. I don't know which batteries they have are 4 cell and which are 8 cell but if you're looking at the micro-start stuff with no BMS you're playing with fire. You can kill it by running it down to far, or by putting too many volts into it. For a lithium battery, the max charging voltage is somewhere around 14.9. Does your regulator keep things below that at all rpm? The good news is, Anti-Gravity uses lithium iron
which isn't as prone to flaming up as lithium ion batteries are. So if you ignore everything I've said and overhcharge one, you'll likely only get some impressive arcing and really nasty smoke as it melts. It probably
won't actually start burning.
A battery with no built in BMS doesn't belong in a bike IMHO. I say that as someone who's melted one down that HAD a BMS built in, but when you completely lose the regulator things get fun. This took over 60 volts to melt down but a '91 Ducati has no problems doing that when things fail. With no BMS, 16 volts could eventually lead to this.