Usually you just slosh a bit of kerosene in, and slosh it out again, and see what comes out.
When what comes out is clean, and it looks clean inside, its good.
Don't forget the final rinse out with some clean engine oil.
If it has a solid inch deep of metal particles/carbon sludge, now set solid, it may be a different matter.
And, you want to preserve the paint on it.....
Tell us where its at ?