Thanks for the suggestions, I actually started with Simple Green, then tried isopropyl alcohol, then mineral spirits, then naptha, then acetone (not all in the same place.) Mineral spirits would be my solvent of choice typically, but it didn't seem to shift the crud.
I might try a light wipe with acetone to break the glaze, then dial it back to the spirits. I don't think the paint will look great and the decals have seen better days, so I may leave it to the next owner to decide. It's a shame that it's too big for me, I'd certainly be keeping it otherwise. I mainly picked it up for the parts for my Super Course.