I am of the opinion that if a grow light or bulb maker does not report a PPFD value, you are left to guess on the suitability of the product. Remember that brightness, as perceived by the human eye, is not relevant when looking for a grow light. Knowing the spectral output would be helpful, but the PPFD is key, and the value is dependent upon the distance from the light source.
If you can, in fact, find good LED lamps that fit in that fixture, I would compare the cost of those to the cost of a new LED fixture with similar light output.
__________________ May the bridges I've burned light my way.
I am glad to see someone is asking this. I did give away the four bulb-four foot fluorescent ballasts but I still have a two-foot single bulb one and wondered if it is better to keep buying the fluorescent bulbs or if there was a good replacement LED bulb for it. It is quite confusing....