know a structural engineer?
edit: nevermind, where I am, sill plates aren't usually made of wood, but if yours was wood could it have rotted away (as opposed to crumbling from structural movement)?
oof
I'd start with talking to a few carpenters and if any of them even hinted that there might be complications due to needing to construct a temporary relief to take the load off that wall so a new plate can be placed, I'd be thinking about talking to an engineer
Some general contractors to give you free estimates. They'll tell you whether it will take a specialist or three or just them. Definitely get multiple quotes.