In order to reenable kovarex after removing NuclearFuel, you need to run

/c game.player.force.technologies["kovarex-enrichment-process"].enabled = true

But in order to run that without disabling achievements, I installed LuaCombinator 3 but game.player was a nil value. You need to run:

game.get_player(1).force.technologies["kovarex-enrichment-process"].enabled = true

if you are in a single-player world.

Posting here in case anybody else would find it useful.

Thanks, useful info

Also, in my seablock world, I also needed to re-enable thorium-processing, thorium-plutonium-fuel-cell, plutonium-fuel-cell, thorium-fuel-reprocessing and deuterium-fuel-reprocessing - wondering what else I might have broken.

I've added a mod startup option to enable Kovarex, which can be used as part of removing this mod