We have a large sauna that has been heated by the good old resistance elements under rocks.

6.4kw 240v (I’m in New Zealand)
The elements are kaput.
Rather than rushing out to replace elements I’d rather investigate newer technology to heat the sauna. Any of you experts on this fabulous site have insight as to what’s modern best practice to heat a large sauna?
3.7 x 2.8 x 2.3 m
That’s a shade under 24cubic meters.

Cedar walls over concrete block structure,very well insulated ceiling.