Hi Stephen and welcome to the forum. The first obvious question is "What are the sails made of".
If they are a plasticised material I'm sure you could simply lay the model down and with the sails on a flat supported surface simply wipe then with a damp cloth with just a little liquid soap in the water.
If they are cloth, which, looking at them seems to be the more likely, then they may be stained and a little more difficult to clean. You can try the same process as above first but I suspect that you won't get all the staining out so you may be left with having to remove them and wash them in a bowl of cool soapy water before rinsing and laying flat to dry out.
Having said that I suspect that some of your marking is not all insect poo, which can be tenacious, but it could also be mold caused by damp. This does need removing thoroughly as the mold will eventually rot the cloth. It should wipe off in place though. If it is mold you want to try to kill the spores with something like an anti bacterial hand spray after you have washed it. Do not use anything with bleach in it though as that will rot the cloth as well.
Whatever you do keep any concentrations very weak and avoid hard rubbing that will deform the weave and change the shape of the sails.