I have a plane whose fuselage interior has has a moderate amount of exposure to exhaust residue, smke oil, and what not.
Now that I've removed the source of the problem, what's the best tactic/product for cleaning up the remaining mess?

Pack it with kitty litter for a few days. That will absorb most of it out and after you remove the litter seal up the inside with fuel proof paint or thin epoxy.

for a easy way to clean the goo out put alcohol in a spray bottle and wipe out with paper towels the alcohol evaporates fast and breaks down the fuel gunk so you can wipe it out easy no kitty litter dust to deal with

I haven't used it, but several people have said the K2R spot remover works well.

K2R works very well. Also corn starch. Cornstarch absorbs the oil in the wood. Put it on, rub it in, wait a few hours and brush it off/out. As with kitty litter, several applications will most likely be needed to get the oil out of the wood.

Once the oil is out of the wood, seal the wood with epoxy, thinned about 10% by volume with acetone. Depending on air circulation and temperature, this will take several days to dry. Another option is to use clear fuel proof dope (Aerogloss).