Not a solution but maybe some help.
Really old spilled diesel has kind of a musty smell with a hint of the original oily diesel stink.
Doesn't really smell like you'd expect it too.
Hope this helps with smell identification.
If you've ever been in the basement of a house with an oil
furnace that kind of musty smell is old furnace oil.
Think I would be looking at scraping the heavey build up then laundy soap and a scrub brush for the remainder. Anything petro chemical will spread it around, when the solvent evapourates you will be back in the same situation except it's spread out more! Let the soap/water run into the bilge
then pump it out. Hopefully the crud stays in suspension.
It will probably take a couple sessions to get it to the point where you are happy.
If you use a pressure washer please make a video! That would be great to watch. Last time I used mine in an enclosed area I ended up covered from head
to toe with blow back! LOL blackface with bright white eye sockets from the goggles.
It will work but it will be dirty!
Got a kid? Perfect job, get filthy and play with machines!
Not sure I would be happy dumping laquer thinner in there and working with it in an enclosed area. Without a fresh air respirator you may come out of there a few IQ points lower than when you went in drooling out of one side of your mouth!