oh ok, careful about moduleEnginesFX as they are tricky to work with compared to the regular moduleEngines. With the NASA engines they are all enginesFX If I recall. They dont work the same way when applying engine effects. But here is my go at it, Go in your config and find this PART section:
@PART[Size2LFB]:FOR[HotRockets] //
{
"lots of stuff"
}
and copy and paste that into the bottom of your NASA config file.
Then swap what is at the very top @PART[Size2LFB]:FOR[HotRockets] //
and replace this line:
[Size2LFB]
and make it
[MassiveBooster]
You will have to play around in smoke screen to get the flames and smoke to line up and to match scale.