I'm just about to replace the PS-1221S back-up battery and was thinking about alternatives.

That battery is rated at 2 Ah but at the 1.3A needed for the ignition it would only last about 30 minutes. I believe that has been confirmed by others.

A small lithium motor sport battery like this could fit in the space with little modification and give about 2 hours. That would be more re-assuring if flying in remote areas.

Has anyone tried something similar or can see any snags with the idea?

It does have a built-in BMS so should be reasonably protected against damage causing a thermal runaway.