I was looking into this some more and had a realization: Deep cycle lead acid batteries have better energy density and cycle life, and pairing them with ultracapacitors (to provide starting current) is more cost effective than lithium titanate if you have a small engine.
For example:
YTX14L battery weighs ~10lbs, 14Ah capacity
This 14Ah battery probably has higher internal resistance, but it weighs 7.7lbs:
https://www.amazon.com/Chrome-Batter...3125925&sr=8-6
350F Maxwell ultracapacitors are 60 grams each and have leakage current on the order of 0.3mA, and a set of 6 is probably around 1lb all in with the balancing board.
100F ultracaps are more like 20 grams each, so this package is probably only 200 grams all in:
https://www.ebay.com/itm/US-CDA-16V-...5/313179060140
I believe you can get away with the 100F capacitors because they'll be fully charged by the battery, which can also provide a few dozen amps of starting current.
The whole package performs almost like lithium titanate that weighs 3lbs more, but it has a cheap lead acid battery that you can replace for 30 dollars.