I'm also using a LiFePo4 battery (illegally) for cranking the engine.
I found a type that is recommended for UL airplanes with the Rotac 912 engine. It has very high cranking max current, practically the same external size (very small mod needed to the battery box, reversible).

It has slightly higher standard voltage (13.9 V IIRC), and the voltage drop while cranking is much much lower than with Pb batteries. So the engine cranks faster and I believe starting has become faster too. For the same reason, the alternator and regulator only charge the battery to (let's say) 90% of full capacity. A few times per year I bring a programmable charger and the old Pb battery to the sailplane, rig it to the battery connector, feed Pb power to the charger, and fully charge the LifePo4 battery. There is practically no self-discharge, so provided I opened all circuit breakers after last flight, the battery is still fully dependable even after many months.

The overall energy stored is inferior to the spec of a new fully charged Pb.. So you have an inferior number of possible attempts to start. But as the voltage remains practically constant even when substantially depleted, I consider it a convenient exchange.

This is also the reason why, with Lifepo4, you should definitely not draw panel power from the engine battery.

Aldo Cernezzi