That sump is a real pain. We cleaned ours out when we repowered and the engine was out. Even then it was very difficult to get all the decades worth of gunk out of there. Belt dust, oil drippings, tools and who knows what all came out of there. After we got it cleaned out, we installed a bilge pump on a stick that sits down in the sump. We still regularly pour in some cleaner.
If you don’t pull the engine/tranny, I’m not sure how you would get down there to clean it. I think you could probably reach it with a long vacuum cleaner tube on a strong shop vac. Maybe repeatedly fill it with water and vacuum it out?
Sorry i can’t offer any better ideas! Let us know if you learn some super trick.
Greg & Lyn
s/v Paperbird
Pearson 422
But if not...