From the data sheet, the Elrog should work in a stock Kaiju, making about 4.8 watts at 5% second harmonic. The data sheet also indicates that DC filament power is recommended.
As Doc B indicated, we have not tested this, so we cannot make any recommendation based on our own knowledge.
When it first appeared, data was scarce and I made some guesses and estimates, which I posted either here or on the Asylum (can't remember exactly). Now that the data sheet and application note have been published, I recommend you ignore those earlier posts.