That is ZERO problem: This CDI is indicated to work from 4,8 to 6 volt, NOMINAL.
That means that it will work on a 4 cell NiMH, OR a 4 cell 1,5V dry cell pack.
These ignitions will without issues run on a 4 cell NiMH, and until empty. I have seen these to work without issues down to below 4V actual voltage.

The early RcExl were nominated that, and 99% sure this is a rebranded RcExl.

The later versions, that had "6V till XX volt" on them (there were varying versions over the years) indeed started to malfunction below 5,5 volt actual, but these ones go much lower.
A modern 2000 mAh four cell AA pack, like Sanyo Eneloop, will easily power that ignition for about 4 hours nonstop operation.
I know because I still have a few of these old ones in use, and those are my GoTo packs...

I usually feed these ignitions straight from the receiver busbar, never had any interference (using 2,4 GHz).