I bet this bike would do pretty good with a 24V lithium pack. Just yank the stock rack and replace it with a Topeak rack and bag with the lithium. Minimal rewiring and you would still come out way ahead, price-wise. You wouldn't see the performance drop during discharge that you see with the SLAs. Figure $300 for the bike + $200-$250 for the battery + $100 for the rack/pack. If I were building a new eBike, this is probably the way that I would go. WAY CHEAPER than my 36v LiFePO4 kit build!
For another $100, you could go 20ah on the battery at less weight than the SLAs. I have a 20AH 36V that weights exactly 20lbs.
LiFePO4-24v-12ah-BATTERY-FOR-ELECTRIC-SCOTTER-