For reaching inside the acess ports, I use a sock with the toe cut off. I pull it up over my bicep on "the in the tank arm". It saves you from those tank edges.
Soap, lots of soap, and water. And a pump to circulate it for a few says then pump out the water. Then more clean water ran thru and pumped out for a few days. Then fan dried after being vacuumed out. Simple.
"Do not treat oily water with detergents. Soaps pollute and make clean up impossible. You may be fined up to $25,000 for using soaps to dissipate oil."
Don't forget the fuel lines and filters they may also be contaminated. You don't want to put all that work in then inoculate the tank first time you use the engine.