Thanks to everyone for the helpful replies. Special kudos to SpyderMike who brings home the coveted thread grand prize (a virtual pat on the back) for the winning suggestion. I just got the Deltran LiFePO4 battery -- holy crap! It's unbelievably small and light, and the price is surprisingly reasonable (no more than a lead acid at my local auto shop). This is exactly the type of modern product I was hoping to find. I've got it on my Deltran charger right now. With any luck, my old Spyder will be back in action this weekend.